Any Human Heart, Sunday, Channel 4<br/>Peep Show, Friday, Channel 4

A big-budget literary adaptation crossing continents, straddling the 20th century and featuring a vast ensemble of national treasures and brightest young things?

Does "quality drama" get more quality than this? The gushing wave of coverage that preceded the launch of Channel 4's Any Human Heart suggested not. And yet after watching last week's opener, "white elephant" was the phrase that more readily sprang to mind. More than 105 long minutes, this pseudo-biopic proved to be as negligible in substance as it was voluminous in scale.

In truth, it always seemed a tall order to make dramatic sense of William Boyd's 2002 novel, aka the "intimate journals" of fictional writer Logan Mountstuart. Journals, after all, deal in the random and the internal, neither qualities which translate easily to the small screen. Still, Boyd himself wrote the screenplay, and has talked a good game in recent weeks, noting that "as the author writing the screenplay you can actually be far more ruthless and audacious". And astute, certainly, considering the book's maxim that "every human being is a collection of selves", was his decision to split the main part three ways, with ailing, solitary octogenarian Jim Broadbent reflecting back on his years as young 'un Sam Claflin and middle-aged Matthew Macfadyen.

That aside, however, the opening episode was not so much creatively ruthless as tawdrily reductive, filleting those aforementioned intimate journals down to a slapdash flick-book of Logan's romantic and sexual conquests. So, his virginal school years were duly done away with and his student years among the dreaming spires became a kind of "Confessions of an Undergraduate", centred upon his dogged quest to get his end away. Within the space of 45 minutes, he had had an affair with his best friend's girl, Tess, had fallen in and out of love with the haughty socialist, Land, and had married petulant aristocrat Lottie.

It left some of our finest young actresses vainly attempted to mug a character out of their hair's breadth of screen time while Logan himself drifted through proceedings as an impregnable blank, in both his Claflin and Macfadyen incarnations. Where the former was prissily stiff, the latter was stolidly lugubrious; and they both lacked the raffish charm crucial to the original conception of him as a philandering narcissist you might possibly root for.

No doubt, things will pick up, just as soon as we see more of the expressively forlorn Broadbent, as well as Gillian Anderson's archly alluring Wallis Simpson. (Oh yes, did I forget to mention Logan's inadvertent encounters with assorted big historical cheeses? All very Forrest Gump.) Though having read the book, I'm not sure whether I need to get the knock-off televisual T-shirt.

When I interviewed Peep Show's Jesse Armstrong a couple of years ago, I asked him whether he and co-writer Sam Bain had thought about quitting while they were ahead. "It takes self-awareness to know when you're past your peak ... we'll probably lack that like everyone else and crank them out until [audiences] are bored of them," he joked. But though the ever-increasing profile of its stars David Mitchell (inset below) and Robert Webb would suggest there's no danger of audiences getting bored with this caustic flatshare sitcom anytime soon, I do wonder whether the new run would do well to be the last.

The problem, to this carping critic at least, is that where its original brilliance derived from the stasis of its lead duo, thirtysomething suburban odd couple Mark and Jez, they have increasingly fallen prey to such conventional dramatic demands as plot and character development. In Friday's hospital-set series opener, indeed, we saw Mark become a dad, a game changer which resulted in an ending of disorientating sentimentality. That's not to say that Peep Show isn't still a lot funnier than most of the competition; only that these are two self-destructive Peter Pans who should never be allowed to grow up.