You don't have to be Sherlock Holmes to solve the mystery of why TV tecs are so popular - and that's why ITV has signed a four-year deal winning exclusive rights to all Agatha Christie's novels.

FOR many of us, the most baffling thing about Agatha Christie whodunits is why these particular murder mystery novels have endured for quite so long. The books are readable enough, although few would claim they contain great literary merit, and their shortcomings are painfully exposed when adapted for the stage and screen.

Characters are revealed as clichs, mere tools for the mechanics of the creaking plot. Even Christie's best-known sleuthing creations, Hercule Poirot and Miss Marple, conform to outdated stereotypes of funny foreigner and prim spinster, rather than a flesh-and-blood person.

There was a period in the 1970s when the cinema adopted Christie. Producers realised that if you put star names in every role, people would not notice that the character was a cardboard cut-out. Murder On The Orient Express and Death On The Nile were spectacularly successful at papering over the cracks with the talent of the likes of Albert Finney, Bette Davis, Mia Farrow, Richard Widmark and Vanessa Redgrave.

ITV lacks such star power, so why have bosses bothered to go head-to-head with the BBC to secure the rights to the Agatha Christie collection? After what was described as a "Titanic struggle", the commercial broadcaster emerged victorious to sign a four-year deal, giving it exclusive rights to dramatise all the novelist's books.

LWT drama has announced that 24 hours of adaptations will be produced. The reason for the attraction to Christie, and you don't need to be Sherlock Holmes to solve this particular puzzle, is British viewers' long-term love affair with watching the detectives. Her work provides a proven and popular source of material to be investigated over and over again.

The new deal will also help some way towards ITV's bid to plug the gap left by the demise of Inspector Morse. Both channels have been searching high and low for someone to take the place of Colin Dexter's Oxford copper. The holy grail is a long-running franchise that can be relied on to pull in the lion's share of viewers. Morse also demonstrated that people will watch the same episodes over and over again, despite knowing the identity of the guilty party after the first time.

So expect to see the return of David Suchet, sporting a wax moustache and exercising his little grey cells, as Belgian sleuth Poirot in a role he's been playing since 1989. Poirot turns up next weekend in a two-hour film left over from a previous batch. This is Evil Under The Sun, which has already been made into a movie with a cast far more star-studded than ITV could afford.

Now the race is on to find the new Miss Marple, played by Joan Hickson in a series of BBC adaptations between 1984 and 1992. The favourite to put on her tweeds and thinking cap is Prunella Scales, alias Sybil Fawlty from Fawlty Towers.

Ironically, ITV might have no need of Poirot and Miss Marple to take over from Morse. Two of its current crop of new TV tecs are doing very well in the ratings. Midsomer Murders is the more established, with policemen John Nettles and Daniel Casey investigating murders in the English village.

Just finished a first run, Foyle's War has achieved excellent viewing figures, justifying the ITV Network's decision to commission the series even before the pilot episode was broadcast. A second series has now been ordered. Michael Kitchen's wartime detective was created by Anthony Horowitz, who's also the writer behind Midsomer Murders and who's adapted Evil Under The Sun.

Both series follow the expected pattern of an experienced policeman, supported by one or two junior sleuths, probing the deaths among a cast of familiar TV faces. The pace is slow, the murders often ingenious but not too gruesome, and the scenery is pretty.

This has more in common with the Christie world of money and privilege than the gritty realism of Waking The Dead or Cracker. Viewers know that, despite violent death and hidden passions, the bad things are happening to someone other than them and that all will be resolved within two hours. Like watching people suffer in soaps, it's comforting and reassuring to see others worse off than you. If the setting is somewhere hot and exotic, or rural and picturesque, so much the better.

The nearest thing the BBC has to Inspector Morse is Dalziel And Pascoe, with Warren Clarke as the bluff Yorkshire detective with the oddly-pronounced name (say "De-yell") and Colin Buchanan as his sociology graduate partner. The series began in 1996 and, although all the supply of Reginald Hill's source novels have been exhausted, fresh cases are still found for the pair.

While the BBC has lost Agatha Christie, it has gained one of ITV's old detectives, poetry-loving Adam Dalgleish from the novels by P D James. Roy Marsden has played the superintendent on ITV on and off since 1983.

He'll be replaced as Dalgleish by Martin Shaw, currently to be seen Judge John Deed on BBC1. What's doubtful is that Dalgleish, and probably not even Poirot, will outlive Baker Street's famous detective Sherlock Holmes. A number of actors have played the pipe-smoking, drug-addicted sleuth on both ITV and BBC, among them Jeremy Brett, Peter Cushing and Douglas Wilmer. Now they're joined by Australian Richard Roxbrugh, who plays Holmes in a new TV adaptation of The Hound Of The Baskervilles being screened on BBC1 over Christmas.

* Poirot: Evil Under The Sun is on ITV on December 15 at 9pm.