Last night's viewing - The Poison Tree, ITV1; Little Crackers, Sky1

Just because ITV is currently running a drama – The Town – that teasingly exploits the overfamiliar formulae, doesn't mean there's no room for the overfamiliar formulae any more. The Poison Tree, the latest of ITV's Monday/Tuesday two-parters, is a good case in point. Get out the I-Spy Book of Thriller Tropes and you'll be ticking off the boxes in fine style within seconds.

Mysterious call in the middle of the night? Tick. Distorted flashbacks to the youthful crime that has come back to haunt our protagonist? Tick. Ominous figure who appears on the horizon but has vanished when you look back again? Tick, tick, tick.

It began with a prison release, Karen and Alice waiting outside the gates to welcome home Rex. We don't yet know what Rex has done but we do know, pretty quickly, that whatever it was is still troubling him. "I want to see the house. I need to," he says as they drive away, overcoming his wife's misgivings in order to pay a visit to the site of his crime and trigger a classic memory sequence in her, complete with echoey voices and heavily filtered hand-held camera shots.

Karen, played by MyAnna Buring, is the centre of the thing, an innocent outsider who finds herself pulled into the orbit of rich, spoiled siblings Biba and Rex, a Cocteausque pair who throw wild parties in their palatial London home and both seem to have a yen for Karen. Within about 30 seconds of meeting Biba, I wanted to murder her myself, and unless she's the person lurking around in a hoodie giving Karen the heebie-jeebies, I think it's a fair bet that someone else eventually got around to it, driven beyond endurance by her kooky bohemian impulsiveness.

That's the basic deal anyway. The past, when something horrible and murderous happened (the flashback contained one of those slowly seeping pools of blood indispensable to such dramas), and the present, where Someone Knows Your Secret. In the present, Karen and Rex live in a clapboard house at Dungeness, which is handy when it comes to not bumping into old acquaintances but also conveniently isolated for lurking, spooking and menacing purposes. Hilariously, it also seems to have a snooty social network. At one point, Karen's neighbour, a yummy mummy in a Chelsea tractor, calls by to complain that work on the beach is "bringing all sorts to the area". Just awful, isn't it? You'd think the nuclear-power station would keep the riff-raff away.

Anyway, by the end of last night's episode we'd discovered that Biba had tipped her disappointing father over the bannisters back in the day (the spreading pool was his) and that almost immediately afterwards Karen had killed one of Biba's louche friends to prevent him calling the police about it. And that Rex might well have served 12 years in prison in order to cover up for something she's done. We'd also discovered that killing the clichés in one section of the schedules offers no guarantee that they won't spring up with flourishing vigour in another part of the garden.

Sky's seasonal Little Crackers series returned with an autobiographical short about a Sixties model, directed by Joanna Lumley, who took a small cameo role herself as a fashion magazine grande dame. Baby, Be Blonde was an innocent affair, as guileless as a teen-magazine photo-romance. It told the story of a Lumley-like ingénue, briefly tempted by the magical glamour conferred on her by her new blond wig (suddenly men are whistling at her in the street) but then rebelling against the oafish sexism of a Baileyesque photographer, in the interests of sisterhood and solidarity. Lumley went through all this at the time, so I guess she's got the details right. But I still found it tricky to buy the notion of the hottest young snapper on the block doing a shoot for a knitting pattern catalogue.