Gavin and Stacey, BBC One

When is enough? The template usually cited as the perfectly proportioned lifetime for sitcom is Fawlty Towers. It ran for two series, 12 episodes - in and out, no mucking about. The Office deliberately kept the same hours, give or take the odd Christmas special and an entire American remake.

Disappearing off the other end of the scale was Only Fools and Horses, which adopted the opposite tack of keeping faith with its characters as the contours of their lives changed. Gavin and Stacey, whose creators Ruth Jones and James Corden are knowledgeable students of the genre, has always had the look of a sitcom which could fit into either camp. The alternative danger was that it might fall between two stools. It returned last night for its third and final series.

Once upon a time, it looked as if that day would never dawn. The first series had a beautifully built-in finality to it: boy meets girl, boys loses girl, boy marries girl. The situation, if you can call it that, was part geographical, part cultural: the boy was from Billericay, the girl from Barry. The comedy was provided by everyone around them: embarrassing relatives (led by Rob Bryden and Alison Steadman), and fat friends (played by the two writers). The oddity was that, while it won its Baftas, on BBC Three most people didn’t know Gavin and Stacey, or their bezzies Smithy and Nessa, from Adam and Eve.

To pave the way for the junior channel’s relaunch, BBC Two set aside three hours for the transmission of the entire series. If you had the stamina, this was a stroke of scheduling genius. Swallowing  the show in one all-you-can-eat chunk brought out flavours rarely tasted in half-hour comedy: a propulsive plot and a capacity to tug unmanipulatively at the heart strings. This was a comedy brave enough to field two main characters, winningly played by Mathew Horne and Joanna Page, who barely raise a titter between them. The last episode even had the courage to terminate on a gag-free downer.

It says much for the show’s popularity, as well as the breadth of its natural constituency, that it was wasted in the yoof ghetto of BBC Three. Having tarried on BBC Two for its second series, Gavin and Stacey has now become the first sitcom to journey through a series of consecutive promotions all the way across to BBC One. We probably have Horne and Corden’s iffy year to thank for its return. Their sketch show did not, in cricketing parlance, trouble the scorers, and then they made a film with lesbians and vampires in the title and no laughs or charm or anything much in the actual script. Corden was suddenly available.

Have he and Jones gone a bridge too far? As Nessa would say, I not gonna lie to you: the second series looked distinctly shaky to start with. It felt like the morning after the wedding, a bit hung over: the worry that the newly-weds might not have done the right thing discoloured the entire jolly romp with a patina of uncertainty. There’s none of that here though. Both main characters being deeply rooted in their own worlds, the second series was all about Stacey’s move to Essex. The third series, naturally enough, is all about Gavin’s move to South Wales, flagged in last year’s Christmas episode.

And that, they have officially announced, will be that. Once Gavin and Stacey have presumably had the baby they pledged to try for last night, the show will have run out of excuses to keep the characters schlepping up and down the M4. What will happen to Nessa can only be guessed at. Her wedding is promised for the final episode. She's currently engaged to the sturdy Dave Coaches, but evidently getting cold feet (in one of last night's courser gags, they don't smell too good either). Who she’ll walk up the aisle towards will provide the Barry Island equivalent of Who Shot JR? Whoever it turns out to be – and because this show has never missed a chance to make you shed a tear, my money is on Smithy - the lucky man will find her virginity reinstated thanks to the wonders of hymen replacement therapy. This despite the existence of baby Neil, who in the latest family gathering was christened.

Much of the comedy in the first episode focused on an Essex boy’s arrival in a country where the soap on TV is in another language, while his new colleagues play seven-a-side with an oval ball and their in-jokes are incomprehensible. “Am I being thick?” said Gavin. “I just don’t get that.” Of course it’s Stacey who is thick as a very substantial plank, for all her implausible new interest in psychology magazines and babble about latent boundaries. Sometimes Gavin gives her the look of someone who can’t quite fathom why he married such an extraordinarily shallow woman. It happened here when she described the global downturn as “the credit thing”. One of Stacey’s mags might make it clear to him that he has simply married a cuter version of his malapropistic mother Pam.

In another life these characters could have grown old in front of our very eyes without trespassing on anyone’s goodwill. Having done an engagement and a wedding, a stag and hen night, a birth and a christening, you'd love to see Jones and Corden have a crack at death and all its trappings, perhaps of Barry Island's resident octogenarian nymphomaniac Doris, whom I'd happily see croak. Instead they're writing their own funeral. Oh well. It was tidy while it lasted.