The only reason there are ladders in Portwenn is so that folk can fall off them but tonight the soft-hearted Joe Penhale manages to get himself stuck up one a few feet from the ground.

He's trying to impress his estranged wife Maggie (Julie Graham) with his ­new-found bravery in the hope that she'll stick around and give their marriage another shot. And, frankly, she could do a lot worse.

Look at Louisa, stuck in a non-relationship with a man who's missing most of the standard quota of emotional ­software. Tonight, Louisa returns to her teaching job, just in time for half the pupils and the caretaker to be struck down by a ­collection of strange illnesses.

But while Martin is handy to have around in a medical emergency, as far as normal relationships go he's a complete waste of skin. And that's not a situation Louisa is finding funny any more.

The Cornish whimsy continues tonight as Martin (Martin Clunes) and Louisa (Caroline Catz) discover that having a baby can actually cause a surprising number of arguments. They tackle his christening (Martin books it without consulting Louisa), who should return to work and whether the child should go to boarding school one day. Meanwhile PC Penhale (John Marquez) seeks to stop his recently unearthed wife Maggie (Julie Graham) leaving the village and school caretaker Mr Coley (Brian Pettifer) goes completely gaga.

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As Louisa and the Doc finally knuckle down this week to naming their poor baby, Julie Graham, who played Martin Clunes's wife in William and Mary, breezes into Portwenn - but this time she's married to PC Joe Penhale (John Marquez).

If your reaction to that is: I didn't know he was even married, well join the club, nor did Joe. But he seems awfully pleased to see Maggie all the same.

It turns out to be another medical puzzler for Doc Martin, whose talent for identifying extremely rare medical conditions rivals even Gregory House's.

It's all the more impressive seeing as the Doc is equipped with just his medical bag.

He usually has to double as a paramedic as well, since the folk of Portwenn have a strange inability to dial 999 - preferring instead to make the poor doctor sprint to each and every emergency.

I do hope that somebody buys him a nice pair of trainers for Christmas.

Another biggish guest star arrives: Julie Graham memorably spent three years as Martin Clunes's screen wife in William and Mary, and here they are again, reunited. Except they're not quite, because in Doc Martin, Graham plays the wife of PC Penhale (John Marquez). That's odd, considering we didn't know he was married. It's even odder that his wife thinks he's only just arrived in Portwenn.

Meanwhile, Cornwall's worst restaurateur Bert Large (Ian McNeice) is deeper in debt than ever, a fisherman keeps fainting, Eileen Atkins desperately deserves more screen time as Aunt Ruth, and Louisa's mother is still causing trouble, above and beyond her annoyingly nomadic accent.

As for the Doc himself, an episode full of the customary, satisfying sight of him being rude to people who fully deserve it has a glimpse of warmth at the end. That it's hard-won makes it all the more affecting - Clunes is brilliant at letting those little flickers shine through.

If you're looking for a bit of escapism then you could do worse than visit the quaint Cornish seaside retreat of Port Wenn in this light - and hugely popular - family drama, now midway through its fifth series. It's easy to see why Martin Clunes's shirty surgeon decided to stay in the idyllic fishing port rather than return to life in London as he nearly did at the end of the last series. The sleepy pace gently washes over you and the magnificent views (in reality those of the village of Port Isaac, which is indeed in Cornwall) transport the viewer to a picture-postcard paradise.

Tonight Clunes's no-nonsense Auntie Ruth (the splendid Eileen Atkins) is on formidable form cheating at chess, berating her chickens, and wanting to "bring back capital punishment for people who use their mobile phones on the train". The programme avoids banality by touching upon some serious medical issues. When PC Penhale's (John Marquez) ex-wife Maggie (guest star Julie Graham) turns up thinking it is April 2008, the Doc quickly spots signs of transient global amnesia and arranges an urgent brain scan.

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Louisa's thoughtless, feckless mother (Louise Jameson) arrives in Portwenn to meet her new grandchild. But granny isn't welcome as it becomes obvious that Louisa and her mum have Big Issues from way, way back in Louisa's childhood.

But that doesn't stop Louisa, who seems to spend a disproportionate amount of time pushing that unwieldy pram around the village, from asking mum - who is smoking a fag at the time - to babysit. Yes, of course, it all goes a bit wrong. Meanwhile the Doc continues to change his new son's nappies while wearing rubber gloves.

The only sane note is struck by splendid Auntie Ruth (the divine Eileen Atkins) who is very gamely making a go of the farm.