With just a few days to go until the animated, restored and thoroughly rejuvenated Power of the Daleks is released, 50 years to the day since it was shown on BBC1 - probably after the football scores and accompanied by egg and chips and a pea-souper at the door. It's a wonderfully evocative thought - and offers a pleasingly circularity to the autistic instincts of Doctor Who fans.
There's something else that's interesting about time here. 2016 will be the sixth year when there's new Doctor Who on television, albeit only once and at the very end of the year this time around when Peter Capaldi will be joined by Pearl Mackie and Matt Lucas in time for another sherry-soaked romp (ugh).
This year we have an anniversary treat (sort of) in the shape of Power of the Daleks, a story whose reputation has comfortably eclipsed Evil of the Daleks (fingers crossed that one will come along later if sales of Power... are promising).
There are good reasons for choosing Patrick Troughton's first story. As the introduction of the Second Doctor, forming a companion piece with the newly-restored The Tenth Planet, it has a cache beyond most lost stories, especially following the welcome recovery of The Web Of Fear. And on its own anniversary there's a ready-made story of its release.
However there's more. Every ten years after Power of the Daleks' original broadcast - give or take a few days, weeks or months - there has been new Doctor Who. And tracking the changes over those decades is a fascinating way of tracking Doctor Who's varying fortunes, contemporary television styles and trends, and the development of what is now undeniably a pillar of television history.
While Power doesn't suggest a radical change of the template from the previous three years, the paradigm shift to episode four of The Deadly Assassin is startling. 11 million people watched as Tom Baker's Fourth Doctor overcame Goth and the wizened new Master.
It's a violent, surreal and often horrifying story - shot in colour, frequently on location and film. For the first time the Doctor travels without a recognised companion and Tom Baker - at the absolute height of his powers - along with Philip Hinchcliffe and Robert Holmes is turning Doctor Who into must-watch television. It's an era rightly recognised as probably the best in the series' 50-year history.
Fast-forward 20 years later and there's another scouse Doctor at the TARDIS controls. Paul McGann's Eighth Doctor fights yet another version of The Master (and American TV executives) in millennial San Francisco, following a seven-year hiatus. Perhaps the most obvious influence on the TV Movie is The X-Files, but the prevailing idiom is apparent in Buffy The Vampire Slayer too.
In all honesty it's a wretched piece of television and a scarcely-recognisable bit of Doctor Who. The idea of a resulting series is fairly grisly in retrospect. Don't believe me? Try, if you can, to read the Leekley Bible without weeping.
Another ten years later and Doctor Who is back on TV screens feeling as confident as it had 30 years before. The Runaway Bride holds little interest for me but there's no doubt The Tenth Doctor's Christmas romp (ugh) has a very strong sense of what it is doing. A scene where the TARDIS barrels down a motorway to rescue a bride (played by perhaps the most popular comedian of the day) being kidnapped on her wedding day by evil robot Santas is practically the perfect evocation for Russell T Davies' Doctor Who.
Perhaps the most intriguing update in this ten-year cycle though is The Trial Of A Timelord. It's also the first of these stories I ever saw, coming at a time when I had abandoned Doctor Who for daring to kill off my Doctor (the Fifth) and present me with probably the worst run of stories in Doctor Who's history.
I still devoured Doctor Who in Target form but Colin's abrasive Sixth Doctor and a dysfunctional production office had ensured television Doctor Who lost me for several years, bar a chance (and chilling) encounter with the first episode cliffhanger of Terror Of The Vervoids.
A recent rewatch of the entire boxset has confirmed that The Trial of a Timelord remains intriguing in Doctor Who history, albeit flawed - in different ways but to similar degrees - to the preceding season. Colin Baker has clearly mellowed his performance from the previous year and it's noticeable that the first time we see Peri and The Doctor they're conversing like normal people. Both leads have lovely moments together and the folly of the fractious relationship in Season 22 is immediately clear. There's also the single-most impressive special effect in the series' history as we see the Gallifreyan space station capturing the TARDIS.
Sadly it doesn't last. Robert Holmes' last complete story is every bit as dull as his first. The Mysterious Planet is limp, ponderous, confusing. It's immediately clear that Doctor Who hasn't made a glorious return. Later in the series we get an enjoyable potboiler in the shape of Pip and Jane Barker's Agatha Christie pastiche and more strong work from Colin, at last the Doctor we know and love from Big Finish, where he delivers easily the best performances of all the Doctors. But it's clear that this story is not the programme's resurrection.
It's only in Mindwarp in Trial of a Timelord where Doctor Who suggests that there's a viable, exciting vision for the show. This is unsurprising as it comes from the only person who could be described as an auteur working on the show at this time. Philip Martin's track record of playing with narrative and convention may not completely translate here, but Mindwarp's unreliable narrator, state-of-the-art effects and tonal shifts between high camp and brutality are, at least, novel.
Colin Baker's prickly Sixth Doctor feels more at home here and the character comes into his own. And the radical tone feel vitals in the prevailing political and economic situation at the time - a convincing example of science-fiction as satire. Mindwarp is thrilling, funny, weird - and shockingly powerful. Needless to say, Martin never wrote for Doctor Who again.
And then there's The Ultimate Foe (the aborted Time, Inc). The loss of this story is small beer in relation to Robert Holmes' premature death, but it does deny us his final vision for the programme he will be forever remembered for. And what a vision it offers. If modern Doctor Who is all about revelation, story arcs and narrative twists, this is where it all starts.
The Trial of a Timelord roughly mirrors the modern show's quarter-year run and its season-long story arc. It also builds to a stunning revelation the series has probably never - and could never better. The Doctor's arch foe, who has spent the previous 12 episodes demanding his life, turns out to be... himself. That this news is delivered by The Master, who has also turned up just in time for the climax of this epic story, accentuates the sense that what we're seeing is something very special. Just image the social networks lighting up at twist upon twist that The Ultimate Foe throws at he audience.
It never really turns out like that, as the story devolves into a confusing runaround. Quite what Holmes' vision was we can never know. Eric Saward's patch-up job at least attempts the kind of pay-off - the Doctor and the Valeyard tumbling the Matrix in mortal combat - the set-up demands but it's understandable than John Nathan-Turner was skittish about leaving the story unresolved, lest Jonathan Powell see an opportunity to close the door on his own bete noir for good.
In The Trial of a Timelord there's an unwitting echo of the template that would become familiar in the 2005 series onward. And it sets off one of the most fascinating narratives in the show's massive 53-year universe.
The story of the Valeyard - created by the Timelords as an amalgamation of the Doctor's darker instincts - is never really resolved but the phenomenon creates space for successive writers in audio stories and original novels to explore. The Seventh Doctor is haunted by the Valeyard and the space created by the Sixth Doctor's unresolved television adventures have created a whole sub-universe to explore this facet of the Doctor's consciousness.
Colin Baker's personal misfortune nevertheless creates one of the most bizarre and compelling strands of the expanding Whoniverse. It's something of a microcosm for the Trial of a Timelord which, although largely unsuccessful, has remained interesting in a way that superior stories arguably have not.
This may or may not excite you but it clearly stayed with Steven Moffat, who essentially re-ran the storyline in the excellent Day of the Doctor 27 years later. I believe there's something interesting to be said about almost all Doctor Who. Even when it fails it often does so brilliantly. Doctor Who fails on its own terms.
We're about to get one of the most superior stories in the show's history returned to us on its 50th anniversary. But we're also 40 years on from one of the show's finest hours, with its most recognisable Doctor. The TV movie is an odd landmark, but a landmark all the same; The Runaway Bride is modern Doctor Who perhaps at the apex of its mainstream popularity. Capaldi's Twelfth Doctor has been an undoubted triumph, with Steve Moffat on a victory lap: Doctor Who on Christmas Day is as much of a tradition in the modern age as the Queen's speech.
But let's not forget The Trial of a Timelord. Mark Gatiss once said that Reece Shearsmith had become obsessed with it. It's not hard to see why it holds a peculiar fascination. It's rare that such a mainstream programme is so odd, so ill-judged has been commissioned - the trial framework hobbles any attempts to make the show more dynamic, saps its pace and undermines its drama. Yet Trial of a Timelord flickers and sparkles with moments of brilliance and a conclusion that has attracted writers like moths to a flame ever since.
That the serial ends with a succession of real-life disasters has given it a meta quality. It's impossible to watch the last of the Sixth Doctor without associating it with Colin Baker's love/hate relationship with the show. Perhaps Colin is similarly ambivalent about his swansong.
In Robert Holmes' last episode we glimpse genius. The stunning nighttime location work; the Doctor at war with his future self; the return of The Master. And the knowledge, in retrospect, that the stakes were so high in fiction and in reality. It feels as if Doctor Who has stumbled across something it can do well, something important and daring. It doesn't last, in fact it's the calm before the storm, but alongside The Power of the Daleks it's right to look back on episode 13 of The Trial of a Timelord. We can reflect on what might have been, but we can cherish 20 stunning minutes of Doctor Who amid 14 weeks unlike any other.
As Saturday night entertainment goes, a profund exploration of grief, fear, suffering and captivity is hardly on a par with pizza and wine. But Heaven Sent wasn't just an all-time classic, with a performance from Peter Capaldi that rivets you to your seat, some of the best direction seen on the series and Steven Moffat's redemption all rolled into one. It was a triumphant return for a show that has frequently lost sight of its own possibilities over the last ten years.
Doctor Who is the only show on television that could do something like Heaven Sent: to command sufficient money and audience attention to get away with it; to play with form and convention as much as it did; to explore the reaches of outer and inner space; dread, loss and despair; moving castles, a sea of skulls and a remorseless wraith. A Grimm fairytale, an MR James short story, a PC puzzle game and a Hollywood blockbuster. It even lifted straight from its own stablemate - the Doctor's mind palace a clear nod to Sherlock.
Doctor Who's magpie tendencies were on full display here - and the show has never felt so confident, so right in its unique ability to do thumb its nose at convention, to write its own script. To go the long way around.
At its most banal these past ten years, Doctor Who has felt like a pastiche of pop-culture references - it hasn't seemed especially different from other programmes and, perhaps, more importantly, from itself. You can draw a straight line between The Shakespeare Code and Root Of Sherwood - or The Lodger and The Caretaker. Modern-day Earth invasion stories and time-travel cats cradles and historical pastiches. Doctor Who has spent a lot of the last decade sending itself up - or serving up warm reheats. But Heaven Sent didn't simply feel like nothing else on television; it felt like no other Doctor Who.
Estates and flat and chips and fleets of flying Daleks and BBC newsreaders and Murray Gold's awful kidult Hollywood music have become a rut for Doctor Who over the last decade: a comfort zone that has become not only a default setting, but a received wisdom: you can't defy the template, they say; you can't put off the casual viewers.
But this sort of thinking is a trap. It forces Doctor Who - that programme of limitless possibilities - into ever-decreasing circles, a room whose walls will only keep closing in. The overall trend of the series has been one of diminishing returns, with adventures into the surreal, uncomfortable or odd - the very things that Doctor Who is known for - more scarce than a two-handed Dapol Davros.
Both of Doctor Who's showrunners have lapsed into these ruts and they are evident in episodes including - but not limited to - New Earth, The Shakespeare Code, Vampires Of Venice, The Poison Sky, Cold War, The Caretaker. Episodes whose only aspiration seems to be to last 45 minutes. Or the patented end-of-series, kitchen-sink gangbangs. Episodes where every last gimmick and lampshade will be deployed to paper over the cracks of a plot that doesn't really make any sense.
Steven Moffat has virtually admitted that his primary motivation when writing Doctor Who is to maintain the attention of people who might be inlined to switch over - an approach that puts me in mind of Magnus Greel having to absorb the life essences of more and more young girls, just to stay alive.
But in supplicating before an audience - serving up the same old tropes, the ones that are familiar and palatable - can never be a sustainable approach. Doctor Who has always succeeded by playing by its own rules, changing, reinventing itself and kicking against expectations. When it's good and daring and different it turns casual viewers into dedicated viewers, into fans. The very people who resurrected the series. How many programmes have the power to do that?
Heaven Sent certainly did. It currently has a 100% rating on Rotten Tomatoes and has been routinely described as one of the best episodes in the show's 52-year history. And it did this in spite of the fact that featured The Doctor alone in an incomprehensible castle, bereft at the death of his friend, in a bespoke torture-chamber. Perhaps because of it. Its audience was good for the slot and Twitter has buzzing for hours afterwards. Heaven Sent was a success by any metric.
It's a modern paradigm that means trying to woo viewers, those viewers who will never like Doctor Who through the modern curse of relatability, is a fool's errand. Russell T Davies' famous aside about the planet Zog is perhaps the most unfortunate observation in modern television and has cast a long shadow over the programme. It's a received wisdom that has closed many doors to the programme and it's based on a whim, a funny soundbite.
The outcome of this is most obvious in David Tennant shouting and Matt Smith waving his arms around and companions fancying the Doctor and Moffat's speechifying and Murray Gold's schmaltz. It's led Doctor Who's makers to only try to make programmes like Doctor Who.
You couldn't have something like Heaven Sent every week. It's a gruelling watch that requires - albeit rewards - attention; it's upsetting, horrifying and slow and features no audience identification figure. Its music is offbeat, weird, dissonant - surely a deliberate echo of the classic show's Radiophonic Workshop synths and it's startling how successful they are in freeing the show of its familiar ambiance. Its direction occasionally reminiscent of Tarkovsky; its chiaroscuro cinematography. A 57-minute one-hander with a 57-year-old lead actor.
All things that have been thought impossible in the modern Doctor Who. No, you couldn't have it every week - but you could have something as different as Heaven Sent every week. It's a story that demands rewatching. Its sheer quality deserves it; as a lesson in what makes Doctor Who vital, it commands it.