Most unexpectedly for those who waved it off in 2000 with John Thaw's swansong episode The Remorseful Day, Inspector Morse has become the TV franchise that keeps on giving. Thaw's original incarnation of author Colin Dexter's opera-loving murder detective lasted from 1987 until Morse perished after a heart attack – his deathbed utterance, "Thank Lewis for me", being words that could also be echoed by ITV after they resuscitated the franchise in 2006 with Lewis.
There have now been seven series of the further adventures of Morse's erstwhile assistant, Robbie Lewis (Kevin Whately), and ITV recently commissioned a further six episodes, quashing the rumour that Whately and co-star Laurence Fox had had enough of tramping Oxford's mean streets. Meanwhile, a further spinoff, Endeavour, starring Shaun Evans as a young Morse and originally intended as a one-off to celebrate 25 years since the first episode, proved so popular that it too morphed into a series.
Four new Endeavour mysteries will now be keeping at least seven million of us occupied every Sunday night between now and the end of April. So how does the Morse franchise keep renewing itself so successfully, and what are the common ingredients to the three series?
Michele Buck, executive producer of both Lewis and Endeavour, has no doubt about the first and foremost factor: "Colin Dexter," she says simply of the 83-year-old author who importantly still owns the copyright to his creation. "Colin reads all the scripts and gives me script notes. If we went off-piste, Colin would bring us back ASAP… he's not to be under-estimated. He might be 83 but there's nothing wrong with that brain."
Dexter inspects the final drafts a week before filming begins, and over the years he has super-vised some very accomplished writers indeed. Danny Boyle, Malcolm Bradbury, Anthony Minghella and Julian Mitchell each wrote a number of Morse episodes, while Alan Plater, Patrick Harbinson and Lucy Gannon are regular contributors to Lewis. Russell Lewis has written all nine episodes of Endeavour and is probably the nearest that the programmes have to being a showrunner. I ask him to pinpoint specifics and, unsurprisingly perhaps, he immediately mentions Oxford.
"The eternal city is a third character," he says. "There's something soft about the light that lends itself really well to English melancholy, which is really what these shows are about. At the heart is a rather unhappy melancholy figure."
John Thaw himself passed away just two years after the death of his fictional creation, and Colin Dexter once told me that he would never allow another actor to step into his shoes. "He's not James Bond," he said. But a younger Morse is something quite different. Shaun Evans, 33, has proved inspired casting as the troubled genius of Cowley police station and, says Dexter, "will be the last person to play Morse."
He's a spikey interviewee, with a set of features that remind me of Swedish tennis legend Bjorn Borg. "Bjorn Borg? That's a new one on me… that's cool," he decides. When I ask him to describe his Morse, he talks about the power of mythical archetypes and the book The Writer's Journey by Christopher Vogler .
When I ask what he thinks are the qualities common to all three series, he turns it right round and asks what I think. Mind you, he has only seen one episode of Lewis, and none of Inspector Morse. "I bought the DVDs but never got round to it, maybe purposively. Why?" he adds before I can ask. "I think there can be a tendency towards nostalgia, and if I was to become a big fan of what's been done previous, I would no longer be serving the generation I want to attract to the show."
The Liverpudlian Evans developed his Southern accent by listening to recordings of Monty Python's Michael Palin. "He's from the North and went to Oxford at around that time [that Endeavour is set]." But he needn't worry about Endeavour wallowing in nostalgia, despite the new series incorporating England's 1966 World Cup triumph. "We wanted to do Endeavour noir with this one," says Russell Lewis. "The hard-boiled meets the cosy. The intention with Endeavour was always to make it slightly more blue-collar."
They have also "Scandi-nised" (as Evans puts it) by hiring Kristoffer Nyholm, Danish director of The Killing, to shoot the opening episode. "We all stood in awe and admiration of The Killing", says Lewis. "But having someone not so much in love with the whole Oxford thing as we are, brought, not a chill, but a bit more distance."
The new series also finds Morse embarking on a tentative romance with a neighbour, a recent immigrant working as a nurse. Was the decision to cast a black actress (newcomer Shvorne Marks) anything to do with the so-called "Midsomer race row" of 2011, when creator Brian True-May was suspended after claiming that Midsomer Murders was "the last bastion of Englishness" because it featured no ethnic minorities?
"I don't think it was as considered as that," says Lewis. "I wanted to get away from the bluestocking pre-Raphaelite girl of the week, and then her character seemed a great way to celebrate the NHS and the contribution that immigrants made to that. Having said that, we're not the Hull Truck Theatre company… we are a whodunit. But the joy of it being this period is that you can look at social history."
Morse veterans seeking reassuring constants can enjoy the music by Barrington Pheloung, the franchise's Australian-born composer. "As soon as his score is on it the show is suddenly there," says Lewis. "It's like the final polish." The fourth episode in the forthcoming series opens with Pheloung's arrangement of Purcell's Nunc Dimittis, whose words are not entirely unresonant given the seeming immortality of the property. "As it was in the beginning, is now, and ever shall be. World without end." And if ITV shareholders wants to add "amen", who can blame them?
'Endeavour' returns on Sunday at 8pm on ITV