Roger and Val Have Just Got In, BBC Two

It's a brave sitcom writer who dares to write a bleakly comic drama, without canned laughter, in which nothing very much happens and where a long-married couple natter away about the mundane details of their lives in the half-hour after they come home from work. But twin sisters Emma and Beth Kilcoyne have done just that, and the result, Roger and Val Have Just Got In, is a thing of quiet beauty.

It debuted on BBC Two in 2010 and, given little fanfare by the corporation, still gained a solid and devoted following, instantly hooked on this delicately woven story about Roger and Val (Alfred Molina and Dawn French). In the first series, as they wittered on about what they had for lunch, the staffroom politics at Val's school, where she is a cookery - or rather food tech - teacher, and the customers in the garden centre where Roger works, we slowly realised there was something missing from their story. As infinitely detailed and neatly dovetailed their conversations were, they were endlessly trivial.

She could take the pressure because, as she said, not everyone can cook a Sunday roast

And while the action moved from kitchen to sitting room to bathroom to bedroom, the door to the spare room remained closed. It was only when we finally entered this room, in episode four of six, that we knew this was a couple caught in a co-dependency of shared grief, where the minutiae of life were discussed in detail but the big issues of pain and bereavement were left unsaid. I shall not easily forget the episode's closing shot of a lampshade decorated with stars and rockets, and with it the realisation that this was a nursery that had never had an occupant - Roger and Val's baby son who, we later learned, had died 18 years before.

The second series, which started last night, took up the story where the first left off. Roger is still fighting his unfair dismissal at a tribunal, and Val has applied to become deputy head. They had just come home from a family wedding and a minor argument had started that morning over a bad smell left by Roger in the hotel bathroom, meaning that Val hadn't been able to make full use of the complimentary smellies - ”I'm just saying, Roger, that I would have liked a bath..." It had clearly been revisited several times on their long car journey home.

At the end of the episode Roger finally opened a handwritten note that had been pushed through the letterbox. He was sent into a sweaty panic by the note, but my guess is that its contents won't be revealed for a while yet, for the Kilcoynes have proved themselves masters of deferred gratification.

If what has gone before reads like a review of a drama rather than a sitcom, then that's probably fair, but this is slow-burn, lo-fi comedy, of the sort that Roger and Val's producer Hugo Blick introduced us to in Marion & Geoff, and which we have seen more recently with Stefan Golaszewski's Him & Her and Pete Versus Life.

But there are scenes played purely for comedy in this acutely observed series; there's talk of Roger and Val's shared love of Little Chefs and they are incapable of walking past the fridge without opening it. And last night Val, preparing herself for her upcoming interview, had fashioned a cardboard box with pictures of the faces of Martina Navratilova, Hillary Clinton and Margaret Mountford from The Apprentice glued on it, to get inside their heads, as it were, and give her strength for the tough questioning the interview panel would give her. But she could take the pressure because, as she said, not everyone can cook a Sunday roast.

I first came across the Kilcoynes at the Edinburgh Fringe a few years ago. They co-wrote the sketch comedy Live! Girls!, which Emma performed with Sam Battersea, and their often surreal and lively comedy couldn't have been more different from this understated work. How much is down to their writing or the superb, naturalistic performances of Molina and French, plus Mandie Fletcher's accomplished direction is open to debate, but between them all they have produced a wryly funny and deeply affecting work.

• Roger and Val Have Just Got In continues on Wednesdays on BBC Two