Alison Graham

It's our last visit, for now, to hopeless Greybridge School as we eavesdrop on yet another of Mr Church's unimpressive chemistry experiments, conducted under the adoring gaze of moon-faced Pat (a silent Julie T Wallace).

But there's excitement on the horizon, at least for the staff, with a school trip to Dieppe, led by French teacher Miss Postern (Catherine Tate) who, oddly, has never been to France before. She doesn't seem able to speak much French, either, as her coachload of uproarious pupils and three male teachers arrive at their accommodation. What happens next is pure crude, rude and ribald French farce involving hotel corridors and gatecrashed bedrooms. And David Walliams has given himself a diarrhoea-related sub-plot that might make you feel queasy.

An unkempt man is found slumped unconscious on Portwenn's beach and is dragged to Doc Martin's surgery by two well-meaning fishermen. The unfortunate gent is woozy and dehydrated though a sharp slap around the face from the world's most curmudgeonly ministering angel soon brings him round, after which the mysterious stranger wants to talk about the Ellingham family, particularly Aunty Ruth.

She's just accepted a job as a Frasier-type radio psychiatrist on the otherwise terrible Radio Portwenn, but her audience isn't quite what she'd hoped for. Yes, it's another busy week in this twinkling Cornish jewel by the sea, a place that refuses to be darkened by the thunderclouds of its doctor's appalling personality. His miserable aspect is counterbalanced by the cheery locals, like the owner of its awful restaurant who is furious at a schoolgirl's review, and its endearingly stupid policeman.

Viewers have taken to Big School (co-written by David Walliams and the Dawson brothers) in droves and they are rewarded by plenty of these crowd-pleasing moments (come on, who doesn't want to watch Philip Glenister drag David Walliams from a set of wall-bars?). But the real scene-stealer is Steve Speirs (he was Ricky Gervais's unwanted "friend" in Extras) as sentimental, self-dramatising Welsh geography teacher Mr Barber. He's prepared his class for an important exam: "I'm looking for the geography teachers of tomorrow." Or so he thinks.

Meanwhile, Greybridge School is transfixed by rumours that the chemistry department's prissy deputy head Mr Church has slept with thick French teacher Miss Postern. The rumour was started by Cro-Magnon gym teacher Mr Gunn, Miss Postern's other suitor and Mr Church's rival. In a big slapstick set-piece Church and Gunn (Walliams and Glenister) wrestle - literally - for Miss Postern's affections.

It's a round titled "Kit and Kaboodle" and Stephen Fry wants to know if there's a use for kitty-litter that doesn't involve cats. Alan Davies tries to be helpful, but his contribution ("In an episode of Jonathan Creek I weed into some cat litter") isn't quite what Fry is after. Ross Noble and Noel Fielding, with Australian comic Colin Lane, can't quite lift the episode off the ground.

But there are some bright bits, including Fry demonstrating martial arts on a pile of three bricks: "This takes extreme focus and extreme pain," says Fry, wincing in agony.

After their disastrous honeymoon last week, newlyweds Louisa and Martin return to what passes for normal life in pretty Portwenn. Louisa (Caroline Katz) is back at school and baby James's new childminder Mel, throws the house into chaos when she's late for work. Martin (Martin Clunes) is uneasy about handing over his precious son to her, largely because she's constantly scratching herself. Martin's hopeless beside manner soon sends the new nanny into a fury and the Ellingham family into chaos just as they are about to entertain guests at dinner. Stand by for some comedy business involving a baby monitor.

Why does the nation love miserable Doc Martin?

I've always been fascinated by its huge success because it does nothing for me, says Alison Graham.

There's nothing subtle about Big School - for instance tonight's episode starts with a fart gag. But maybe that's why audiences enjoy David Walliams and the Dawson Brothers' cheerful, unsophisticated creation. Aged gags are telegraphed and thoroughly milked for every single comic possibility. Like the unfortunate pupil at Greybridge School, whose mother has run off with a Masai warrior during a family holiday. Cue sniggering staff making pointed and very lewd remarks (particularly grotesque headmistress Ms Baron) as thick Miss Postern ties herself in knots trying to be politically correct.

Elsewhere, Mrs Klebb presents the school play Juliet and Romeo, her "gender re-imagining of Shakespeare".

There's bound to be ribaldry in an episode titled Knees and Knockers so lie down on your antique fainting couch right now as Stephen Fry and the teams get blushingly saucy. But it's all good fun and even educational. Come on, don't tell me you're not curious about where in the human body the "end-bulbs of Krause" are? Or the pores of Kohn? (Clue: it's not the title of a Star Trek movie.)

Elsewhere, David Mitchell has one of his Would I Lie To You?-type comic rants, this time about, of all things, the supposed idiocy of pandas. We learn why robins are associated with Christmas, the rules for the driving of cars in early 20th-century Pennsylvania and why red kites are called red kites, even though they aren't red.

Pat and Cabbage are 60-something friends. Pat (Barbara Flynn) is a bit staid, Cabbage (Cherie Lunghi) is devil-may-care. Pat's daughter thinks Cabbage is a bad influence on her mum. But Pat gets caught up in her friend's schemes. A painfully jaunty soundtrack accompanies every single telegraphed gag - Pat's daughter brings a hamster into the house, Pat is afraid of hamsters, you know what's going to happen next.

Pat and Cabbage is written by actresses Amy Shindler (Brenda Tucker in The Archers) and Beth Chalmers and is produced by the people who gave us Last Tango in Halifax. But Pat and Cabbage is no Last Tango in Halifax.

It's two years since we last saw flinty curmudgeon Dr Martin Ellingham and when we return to the pretty Cornish village of Portwenn nothing has changed - he's still a miserable git. Even the prospect of his much-anticipated wedding to his beloved Louisa doesn't put a song in his heart. The couple's first attempt at matrimony fell apart, but at last they are successful (this isn't a spoiler, the nuptials are in the opening minutes).

The ceremony is smartly dispatched by a very impatient groom and we spend the rest of the episode with the happy couple (Martin Clunes and Caroline Catz) as they confront a string of disasters on a hopeless honeymoon. There's not much of a story, just a series of increasingly weird and barely credible incidents. Back home Aunty Ruth (splendid Eileen Atkins) is babysitting, and has her fair share of troubles, too.