Cast Offs, Channel 4Gavin and Stacey, BBC1

When characters mock each other's disabilities, we're not sure if it's OK to chuckle. No such worries on Barry Island

Brash guitar music blares in the background as we are informed, via shouty, capitalised intertitles, that "18 months ago Channel 4 marooned six disabled people on a remote island ... our cameras followed them during their time on the island and in the year leading up to them being stranded. Will they survive as ... Cast Offs?".

For anyone who caught Britain's Missing Top Model, last year's excruciating reality show that pitted disabled women against each other for the doubtful prize of a career as a fashion model, the Cast Offs format seems dismally credible. However, few people can have been taken in, even momentarily, given the interest that this mock-doc has attracted in the run-up to its broadcast.

Cast Offs is Castaway meets the aforementioned BMTM: six fictional characters with different disabilities ranging from blindness to paraplegia and dwarfism are challenged to live self-sufficiently on an island off the coast of Britain. Each episode traces the back-story of one of the characters through flashbacks while keeping track of the present-day goings-on among the group.

It's not that this is the first time we have seen disabled characters on our screens – TV and film are peppered with tragic victims and sage martyrs, firmly defined by their respective disabilities and almost always portrayed by non-disabled actors. Which is why Cast Offs – a show written by, acted by and about disabled people – has been hailed as landmark television.

But while the involvement of individuals with real-life disabilities is itself a milestone, it's the licence that this gives the programme to tread some ultra-sensitive ground that makes it such an exciting prospect. The occasional drama has approached the subject of disability thoughtfully, but the idea of placing it within a darkly comic frame remains contentious.

With this in mind, I was prepared for my moral compass to be set spinning – if it's OK for disabled people to crack jokes about each other's disabilities, is it OK for me, a non-disabled person, to laugh at them? It was both a relief and a disappointment that I never had to confront that question fully, because too little of it made me chuckle.

The segments on the island are the weak links, a problem since this is where the interaction between the main characters occurs, giving rise to the supposedly "edgy" humour as they squabble. But while the spectacle of a deaf woman goading a blind man for his inability to master sign language or telling a dwarf that her lips are too small for her to read makes a good point about tolerance within the disabled community, for me it wasn't funny enough to conceal the skeleton of the sermon beneath.

The at-home scenes worked better, sending up the misplaced concern and breathtaking ignorance of the surrounding able-bodied characters. A particular treat was Tom, a blind struggling actor, coaching a pompous RSC buffer to play Gloucester, post-eye-gouging, in King Lear. ("How would you look if you were standing on a cliff? Would you know you were on a cliff?" asks the actor. "Well, yes, after I'd fallen off it, I might think 'that's probably a cliff I've just fallen off'," shoots back Tom.) As the thesp fluffs his lines, Tom takes over the role with a quiet, moving delivery.

The potential strength of this absurd interlude is its stealth. A brief spark meets touchpaper and suddenly the most monolithic of ideas – in this case that blind people are possessed of some magical sixth sense – begin to crumble. So when Tom's love interest later asks plaintively "Why aren't you playing that part?", it dulls an already beautifully made point.

Cast Offs is an important moment in the intertwined histories of television and collective attitudes towards difference. But its value is as a cultural artefact rather than a piece of comedy drama. It's a shame, because there is the potential here for it to have been both. Edgy comedy is something that Gavin and Stacey, meanwhile, has never purported to be. At this point I should probably declare, for those who haven't deduced my nationality from my name, what may well be a genetically predisposed affection for the show.

All the same, I was interested to see whether the gentle BBC series, which returned last week for a third and final series, would have shed a little fairy dust in the aftermath of the lamentable solo efforts of James Corden and Mathew Horne. It didn't take long, however, to be reminded that neither actor has ever been a main draw among the superlative cast (though credit goes to Corden as co-writer). The action has shifted to Barry Island, which will please fans of Ruth Jones's brilliantly deadpan Nessa and Rob Brydon's Uncle Bryn – a caricature, but an excellent one.

The christening of Nessa and Smithy's son provides the excuse to lure the Essex contingent over the border, and the seeds are planted early for what promises to be a warm and fuzzy finale. No surprises perhaps, but for the home straight, I'm perfectly happy with more of the same.