FITTINGLY, Jonathan Creek's first murder inquiry north of the Border is characterised by a spooksome air of visual trickery. For there'll be something curiously mirage-like about the Scottish castle under investigation by the duffel-coated detective with the curly coiffure in next week's feature-length festive TV puzzle, entitled Satan's Chimney. There's no doubt that the Creek film team spent four days on location in Bridge of Orchy. It's also true that the turrets of Glamis Castle loom convincingly over the baronial pile wherein the show's serio-comic mayhem unfolds.

But, in fact, the castle we'll see on screen is a piece of electronic alchemy. It was created at the behest of Sandy Johnson, the Scot who has directed Jonathan Creek almost from day one. ''For plot reasons I can't reveal, we needed a medieval Scottish castle with fanciful towers, plus a moat

and drawbridge,'' the Drymen-born Johnson explains.

''We actually filmed at Raby Castle, in County Durham, which doesn't have any of these features. So they were created via digital artwork by special-effects expert Jim McCarthy, another Scot, who, like me, was a graduate of Glasgow School of Art. Then we electronically placed the whole thing in the valley of Loch Tulla, north of Bridge of Orchy. It looks very real, I'm proud to say.''

Continuing a real film-fiction tradition begun by Alfred Hitchcock, Johnson will make his usual fleeting Jonathan Creek screen cameo appearance. An ex-actor, Johnson's film debut gave him a taste for the eye-deceiving magic of celluloid. ''Monty Python and the Holy Grail was partly filmed in Scotland in 1974 when I was a student, and I played three roles in it. I was a villager, a musician, and one of the knights who says 'Ni!'. I've a long TV CV, from The Comic Strip Presents . . . to Frost, Morse, and Adrian Mole, but Monty Python remains the only thing I've done that impresses young crew members.''

Johnson remains impressed by many factors in the Jonathan Creek equation. ''As Creek's creator and sole writer, David Renwick is a genius. As the show's executive producer, David is also a joy to work hand-in-hand with on

creating Creek's Gothic house style - dark drama done in a funny way, something that suits Alan Davies perfectly.''

Unsurprisingly, David Renwick is another member of the Alan Davies appreciation society. ''He's incredibly effortless-seeming in his acting,'' says Renwick. ''He just is, he doesn't act, something that was incredibly evident in Bob and Rose, in a role that stretched him more than Creek does.

''All the words are fixed in Alan's head, and then he walks out and makes them sound spontaneous - plus he gives them an extra resonance that stops the words being dull. And on top of that, he's funny - a unique quality that can't be learned.''

As Victor Meldew's sire in One Foot in the Grave, Renwick knows all about funny, of course. ''I aim for plausibility all the time in Jonathan Creek, but there's a fine distinction between the probable and the possible - and Alan is the audience's conduit to this somewhat contrived and exotic world,'' says Renwick.

''As Jonathan, Alan is normal, like you and I. He can say, 'That's preposterous!' thus echoing Victor in being disbelieving of the unbelievable events I've constructed around both characters. My original TV inspiration for Jonathan Creek was Columbo, and I'm also a fan of the humorous puzzlers written by fifties crime novelist John Dickson Carr.

''Columbo always began with some incredibly elaborate feat of engineering by the culprit - wires on a turntable linked to a gun, setting it off when the record reached its loudest point, say.

''But everything in Jonathan Creek acts in accordance with the real personality I've firmly established for Jonathan. He makes magic tricks for a living, but he isn't deferential to his magician employer, for instance; he doesn't kowtow.

''It's a relationship that compares, I suppose, with the one I had 30 years ago with various comedians when I was edging away from four years as a junior reporter on my local paper, the Luton News, and writing sketches and gags for radio shows like Week Ending and The News Huddlines, or Ronnie Corbett's weekly monologues on The Two Ronnies.''

Renwick is rare in having put words of enduring mirthfulness into the mouths of not one, but two Scottish comic legends: not only Ronnie Corbett, but also Richard Wilson as the curmudgeonly Victor Meldrew. David Renwick - you're plainly of Scottish descent, laddie.

''I've got no Scottish forebears that I'm aware of,'' says Renwick in his undeniably Lutonian tones. ''In fact, I pronounce my surname 'Ren-Wick,' something that Richard and Ronnie would both pull me up about.''

Nor is David Renwick currently driven by an unstinting auld Scottish Presbyterian work-ethic. Having completed last year's final edition of One Foot in the Grave and next week's Creek special, Renwick hasn't written anything since February - and he isn't ashamed to admit it.

''The BBC are keen for another series of Jonathan Creek, and I'm keen enough, too, but it's a 12-month job, and I'd like to do something else. But I don't yet know what that something else is - and if it's not there, there's no point in forcing it. I was lucky in that half-way through Grave, I was nursing the idea of a detective series - Creek.

''Maybe it's about emulating American themes. One Foot in the Grave anglicised the kind of New York Jewish angst that Neil Simon did in plays like The Prisoner of Second Avenue, while Creek did the same with Columbo.

''Ideally, I'd like to do a British version of Larry Sanders, but it's been done already - and not too long ago - as Bob Martin, with Michael Barrymore. Then again, I've come to learn that the whole TV relationship between the US and the UK is bizarre, and that anything can happen.

''I was out in Los Angeles not that long ago when One Foot in the Grave was being transformed into Cosby, starring Bill Cosby in Victor's role. One of the men involved in Grave's Americanisation was Denis Klein, the co-creator of Larry Sanders. He'd just been approached by Henry Kissinger's son, who'd bought the rights to a British TV show that he reckoned Klein might want to work on for US consumption - Bob Martin.''

At long last! A riddle that even Jonathan Creek couldn't unscramble.

Pledging there'll be ''no snogging, only mystery-solving'', Alan Davies returns as Jonathan Creek in a two-hour Boxing Day special on BBC1 at 9.05pm. In the absence of his usual sidekick, Maddie Magellan (Caroline Quentin), Creek's investigation is elbowed along by forceful theatrical agent Carla Borrego (Julia Sawalha). The pair ponder the strange case of the woman killed by a bullet that apparently passed through a closed, undamaged window.