It’s so barmy that you want it to be true. The plot of this gimmicky British romcom hinges on a 200-year-old law that – supposedly – permits marriage to a dead person. (It was introduced, so it is said in the movie, during the Napoleonic wars to allow women to marry sweethearts who died on the battlefield.) There’s even a word for it: necrogamy. And Mrs stars glorious Aisling Bea as Gemma, who launches a legal bid to get hitched to her American fiance Nathan after he drops dead at home in east London. Tom Hank’s son Colin plays the corpse – or ghost, since Nathan pops back for the odd chat with Gemma (he also appears in flashbacks).

The upside to casting Bea in a comedy is that she’s an absolute hoot. When Hollywood stars play ordinary civilians, there’s often a slumming-it quality to their performances, but Bea is funny and real, sarky and very likable as Gemma, who’s feeling guilty after Nathan dies. He was a big daffy romantic while she never really believed in marriage – and used to joke their wedding was a visa scam. So, in the depths of grief, to prove she really meant it, Gemma decides to go through with the wedding. First, she must ask permission from the lord chief justice (played to frosty posh perfection by Harriet Walter).

But if there’s a downside to casting Bea, it’s a reminder that few scripts can match the quality of her own Channel 4 sitcom This Way Up. Here, the humour is less raw, more calculated to deliver warm, fuzzy feelings. And director Daniel Reisinger can’t resist giving east London a bit of the old Richard Curtis treatment. Still, there’s a knockout supporting cast – including Billie Lourd (daughter of Carrie Fisher) playing Nathan’s trippy-hippy sister and Susan Wokoma as Gemma’s best friend – to stop the sweet stuff getting stuck between your teeth.

• And Mrs is on digital platforms from 2 September.