Hampstead has become quite a hit-factory since Ed Hall took over. His foreign policy is admirably simple. He scours New York for popular shows and spirits them over to London. His latest effort, Cost of Living, has attracted the film-star talent of Adrian Lester, who plays Eddie, a loquacious white trucker from Utah. (His ethnicity is made clear in the dialogue and the relevant lines have been left unchanged.) Earnest Eddie tells us about himself in a 15-minute monologue at the top of the show. Rather a clunky device. He’s a bookish teetotaller with a strong work ethic who appreciates the landscape of Utah, enjoys listening to Erik Satie’s over-played Gymnopédies, and spends his evenings and weekends caring for his -crippled ex-wife who lost both her legs in an accident.

How real is Eddie? A sensitive, noble, charming, do-gooding redneck who doesn’t drink? He feels like a midwest fantasy figure invented by an east coast sentimentalist. Lacking flaws and rough edges, he’s hard to warm to, and although his tenderness to his wife (doomed to death, we learn, in the opening lines) is rather touching, it’s also a bit tiresome and static.

Luckily there’s a secondary drama to enjoy in this ill-structured play. We dart between Eddie and two unconnected characters who are also struggling with disability. John is a rich, snobbish and rather likeable young cripple. Jess is a bombshell cocktail waitress hired to shower and shave John every morning. Their wonky relationship, fraught with danger and temptation, feels wholly original. It’s fascinating to watch the power-play between the prickly, cerebral John and the sexy but tough-minded Jess. All the wealth and the authority reside with the master of the house and yet his diligent maidservant has the right to strip him naked each morning and to carry his helpless frame into the bathroom where she applies her sponge to every inch of his frail but still libidinous body. Their friendship develops — perhaps a little too fast — into a Friday night date. Jess shows up dressed like a million dollars but John has a surprise in store. And their tryst falters. So does the play. The final scenes bring Jess and Eddie together but the drama lacks oomph because we already know more about them — their quirks, their tastes, their essential goodness — than they know about each other. Will they fall in love? The script expects us to supply the answer.

This isn’t a bad show and there are terrific performances from Jack Hunter (John) and from Emily Barber (Jess), a fabulously steely sex bomb who looks like a star in the making. Cost of Living earned the writer, Martyna Majok, a Pulitzer Prize and it’s likely that this socking great gong will render her impervious to advice for ever more. But she’d be wise to include an interval in her plays. A short break improves the emotional shape of the evening and it boosts the theatre’s bar-receipts.

The writer of My Dad’s Gap Year (great title) uses the same uninterrupted format. We meet Dave, a posh waster who drinks vodka for breakfast and sponges off his successful wife, Cath. She’s keen for their teenage son, William, to become a prosperous yuppie. But Dave plans to turn the lad into a derelict alcoholic prat like himself. ‘You’re my biggest failure,’ he scolds. ‘You’re your own worst enemy,’ responds the priggish boy. They patch up their differences and jet off to Thailand in search of booze, drugs and self-discovery. Whereupon the play divides into three. Dave embarks on a semi-Platonic relationship with a mumsy and abstemious transwoman who runs a ladyboy club. William is seduced by a part-Spanish, part-Thai architect. And Cath, meanwhile, monitors her menfolk’s antics from a distance. It’s all good superficial fun.

The plot turns on Dave’s claim to be -terminally ill, which Cath suspects is a hoax designed to elicit sympathy. These are very dark topics to introduce to a sex-comedy. And Dave’s feckless character is hard to engage with because he’s played by Adam Lannon, slim, youthful, well-toned, who can’t hope to portray a fat, elderly juice-head with an unexplained interest in ladyboys.

The iffy staging by Sarah Beaton doesn’t help. The performing area is a slippery white platform with a huge square hole in the middle. This vacancy represents a swimming-pool some of the time, and a safety hazard all of the time. On press night the nimble actors avoided pratfalling into its gaping maw but the risk remains. The show’s best asset is Michelle Collins, who spends much of the drama removed from the action. When she finally arrives in Bangkok she adds a note of humanity and healing to this laddish tale of vodka, weed and grubby sex. The real trouble is that none of the characters has a mission whose success or failure could stir our hearts.