The freewheeling improv style of Jamie Adams (who directed the daffy comedy Black Mountain Poets) hits a wall in his new film about a family Christmas in Wales. There are comedy scenes here that flatline and lightweight fake-feeling emotional moments. Model-actors Suki Waterhouse and Poppy Delevingne are the stars: they look pretty uncomfortable playing sisters visiting their folks for the holidays. Maybe it’s snarky to say, but with their gorgeous knitwear and expensive London accents, it’s hard to buy either of them dunking a Bourbon into a cuppa.

Waterhouse is Iris, a flaky-quirky, beret-wearing film composer who has landed a massive gig writing the soundtrack for a big-time Hollywood movie. The trouble is she’s creatively stuck, and anxious about her mum who is dying of an unspecified terminal illness (fading gently propped up on pillows, no fuss). Iris’s sister, Abigail (Delevingne), is back too, and the suggestion is that this is their last family Christmas together.

To cure Iris’s writer’s block, her agent flies in legendary American songwriter Eleanor Roberts (Dirty Dancing’s Jennifer Grey), who shows up clanking Jack Daniel’s bottles and jangling silver bangles. The two women connect instantly. Cue earnest lines (“You’ve got to be more than a pretty girl with a guitar”) and boring scenes of them writing songs. Eleanor is also clearly attracted to Iris.

Bittersweet Symphony is a film with serious plausibility issues; none of the characters come across like real human beings. But it deserves points for a funnyish subplot that brings home how romcoms so often idealise stalkerish behaviour. Submarine actor Craig Roberts plays Iris’s idiot ex, who devises a series of stunts to prove how much he still loves her – like wrapping himself as a giant Christmas present. It definitely adds to the film. That said, the Onion did the same thing in a one-sentence headline: “Romantic-Comedy Behaviour Gets Real-Life Man Arrested.”

• Bittersweet Symphony is released on 12 April on digital platforms.