The Weekend's Television: Doc Martin, Sun, ITV1 <br/> Merlin, Sat, BBC1 <br/> Spiral, Sun, BBC4

"If we can keep it going imaginatively, without just trotting it out, I think it's worth it," Martin Clunes said recently about Doc Martin, now in its fourth series.

As a very irregular viewer of ITV1's home-grown grumpy doctor drama (more Bungalow than House), I'm not well equipped to comment on its current gait, but I would have said that last night's episode counted as a canter at the very least, rising to a gallop for short sections. To be frank, I'd expected nothing more exciting than a carthorse trudge, this being one of those Sunday-evening calendar dramas, full of postcard wideshots and characterful locals. But then it kicked off with the arrival of Doc Martin's old flame Louisa, six months pregnant and knocking on his door to find another old flame with her feet under the kitchen table. "Don't think you have to rescue her," said Edith sternly, her medical training clearly having equipped her to make a rapid diagnosis of Doc Martin's symptoms, which include sweaty pallor, stammering and a bilious expression. But, since Louisa and he go way back and Edith is a tiny bit daunting in her manner, one imagines that rescuing her is precisely what's going to happen in the long run.

In the meantime, there's plenty to while away the hour, the chief running gag being the complicated interplay between patient confidentiality and the everybody-knows-everything intimacy of a small village (also a theme in Jennifer Saunders's Jam and Jerusalem, which is a composition in the same key as Doc Martin). Doc Martin has a receptionist who tends to giggle at the more lavatorial complaints of his patients and the doctor himself is not always as discreet as he might be when checking up on his patients. "How's that constipation coming along," he bellowed at the local headmaster, when he happens across him in the school. The headmaster, as it turns out, has more to worry about than sluggish bowel movements, since he's suffering from porphyria (full marks for all viewers who'd seen The Madness of George III and shouted out the diagnosis as soon as we saw the blue urine specimen). His derangement peaks when he takes the junior children on an unplanned field trip to a nearby beach, orders them to scrub the rocks clean of seaweed and then marches fully clothed into the sea, which is the sort of field trip one can treasure for life.

Even more startling – for a first-time viewer – was the revelation that Uncle Jimmy's splenetic outbursts were actually roid rage, brought on because he'd been secretly topping up his testosterone levels with anabolic steroids. Doc Martin assumed that this was because Jimmy and his wife had been unable to conceive and Jimmy had been indulging in a bit of ill-informed home doctoring, but then Jimmy fretfully confessed to getting carried away while sharing a tent with a fellow coastguard colleague while on a refresher course in Pengelly Bay and asked whether the doctor can cure homosexuality. His wife thinks he's bi, he explained tearily, but he thinks "bi is just homos wanting to say they're not". I don't know whether this is an accurate representation of the sexual politics of north Cornwall... but I have a feeling Jimmy is eventually going to come to terms with being the only gay in the village.

Merlin is back too, provoking in me an awkward question. I know Uther Pendragon is very enchantophobic and that Merlin can't come out of the closet as a fully fledged wizard, but surely we could have a bit more magic per programme. This week, Arthur was the target of an assassination attempt by a hired killer with a very fancy arsenal of customised crossbows – one with a telescopic sight and another handy pistol sized affair for close work – and Merlin spent most of the episode moaning about the enormous number of chores he had to do. I happen to know, from careful study of such canonical texts as Bewitched and Mary Poppins, that this is not an insurmountable problem for the magically gifted, but although we were shown one almost subliminally short scene in which Merlin had enlisted his powers to help out, he mostly just plugged away at it the hard way, to the extent that he fell asleep mid-scrub. Either he's stupid or he secretly likes hard labour – but in either case he should stop complaining. Guinevere got snogged by Arthur this weekend, incidentally, so if you heard a low sustained groan from the small boys of Britain at around a quarter past seven on Saturday evening that would have been it.

I haven't a clue what's going on in Spiral, pitched in some quarters as the French Wire but the confusion is very stylish. The subtitles are great incidentally. It's actually just a fairly ordinary police serial with a bit of Gallic polish, but if you squint you can pretend to yourself that you're watching Godard.