This year's model a chip off the old Morse

Shaun Evans steps into the shoes of the late John Thaw.

One of British television's favourite detective shows now has a prequel. Gerard Gilbert reports

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 British channel 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). 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.

There are now a series of Endeavour mysteries screening on Saturday nights on TV One. 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 underestimated. 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 supervised some very accomplished writers indeed. Danny Boyle, Malcolm Bradbury, Anthony Minghella and Julian Mitchell each wrote a number of Morse episodes. Russell Lewis has written all nine episodes of Endeavour and is probably the nearest that the programmes have to being a showrunner. Ask him to pinpoint specifics and, unsurprisingly perhaps, he immediately mentions Oxford.

"The 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.

The actor 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."

He needn't worry about Endeavour wallowing in nostalgia. "We wanted to do Endeavour noir with this one," says 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."

What: Endeavour, The Inspector Morse prequel
Where and when: TV One, tonight, 8.30

- TimeOut / Independent