The Weekend's TV: Zen, BBC1<br/>Eric and Ernie, BBC2

It was an Ancient and Modern head-to-head on Sunday evening, as far as detective fiction went.

Over on ITV was Agatha Christie's Marple, in which the poison is generally served up with sweet sherry and antimacassars, while on BBC1 we had Zen, the first of three feature-length adaptations of Michael Dibdin's admired novels about a Rome detective who, on this evidence, works in the Couture Investigation Department, a branch entirely staffed by male models in excellent suits. Zen introduced itself with a stylish set of Sixties titles (complete with the noodling female vocals that became a benchmark of la dolce vita around that time) and looked initially like an extended version of one of the thousands of perfume adverts that infect the airwaves at this time of year.

Well, not quite "initially" perhaps, since only the most challengingly macho fragrance would sell itself with a scene in which a judge is murdered on a remote country road, but once that violent preamble was out of the way, it was pretty much L'Uomo Vogue all the way. The cinematographer appears to be Caravaggio, the costume designer Armani and the chief preoccupations of Zen's colleagues seemed to be extra-marital affairs, long lunches and this season's coolest sunglasses.

The violent preamble was part of a vendetta, an ex-con with a grievance working his way through the list of people he blames for his false conviction, a list that concludes with Zen himself. We, meanwhile, understood that there must be some mistake here because Zen's infuriating probity is a persistent nuisance for his boss (the excellent Stanley Townsend), who has the thankless task of ensuring that the outcome of investigations match up with the state's requirements. And in this episode Zen – played by Rufus Sewell – found himself in a particularly tricky spot. A shadowy figure from high in the ministry wanted him to exonerate a man accused of murder, in case inconvenient facts emerged during his defence testimony. But his boss simultaneously wanted him to make the conviction stick, to avoid embarrassing his immediate superiors. In both cases Zen would pay for failure with his career. He was so preoccupied that he even temporarily forgot to flirt with the gorgeous new secretary, Tania, whose immediate sexual prospects were the subject of an office sweepstake.

It looks good – the images are as sharply defined as the tailoring – and Sewell captures some of the character's melancholy (the result of an ailing mother and a broken relationship), but for anyone relishing the cynical skulduggery of Italian ministerial politics the later plot developments were something of a disappointment. Heading to the scene of the crime, which offered a classic sealed-room conundrum, Zen only had to mooch around in the shrubbery a while before he'd discovered the local wild child and the network of secret tunnels that were the key to the mystery. A drama that began with Berlusconi-style conspiracies, as dark and intense as an espresso, suddenly turned into Enid Blyton with an Italian accent, complete with swarthy villains and underground adventure. Even Zen himself sounded embarrassed: "It's incredible," he told Tania, "the case just fell open in front of me." With any luck next week's episode will get him back in Rome where he belongs, no more than five minutes from the nearest Versace outlet and some nine-to-five Machiavelli.

I hate that title card which tells you that while a drama is based on a true story, some details have been changed for "dramatic effect". What it means, essentially, is that real life wasn't felt to be exciting enough and it always leaves you wondering about the status of what you're watching. Something of a tribute, then, to Peter Bowker's script (and to an excellent cast) that thoughts of authenticity evaporated fairly quickly as you watched Eric and Ernie, his account of the early career of one of Britain's best loved double-acts.

Bowker's drama was actually a three-hander: Eric's mother, Sadie, didn't get title billing, but turned out to be the core of the thing, a woman whose determination to get her son into showbiz overrode his own indifference. "Never mind, son," Eric's father consoled him as he returned from a talent contest brandishing the winner's trophy, "'Appen you'll lose next time." In that exchange you got a sense of an ordinary boy helplessly caught up in someone else's ambitions; for Sadie, though, this wasn't a career by proxy but simple maternal concern: "You make people laugh," she told her reluctant star, "you're a lovely dancer and you can hold a tune... but more than that – and I mean this as the mother that carried you and bore you and raised you – you aren't any good at anything else." She saw that it was showbiz or nothing.

The partnership with Ernie began in rivalry (fighting over the blankets in a reluctantly shared bed) and then mellowed into a friendship, occasionally tinged with envy or resentment but only suffering one long rift, after their first disastrous foray into television, which was played here as a capitulation to metropolitan arrogance and a betrayal of their own comic instinct, painfully developed on a music-hall circuit of grotty digs and merciless audiences. A betrayal, too, of Sadie, whose shrewd advice was temporarily set aside. In total, six performers played Eric and Ernie, and not one of them let the others down – though the Erics, in the drama as in the original act, seemed to have a lot more fun.

t.sutcliffe@independent.co.uk