Standout performances from Laura Linney and Julie Walters distinguish this gentle drama about a youngster awkwardly stumbling into adulthood.
Linney plays a vicar's wife, both pretty and prim - imagine a Stepford Wife who's discovered God - who undermines her husband and rigidly rules her 17-year-old son Ben (Grint).
Walters portrays faded actress Dame Evie Walton, a virtual Hampstead recluse with a drink habit and foul mouth who takes Ben on as a general help during the summer holidays.
However, he is soon much more as he finds himself duped into taking her on a camping trip and acting as a chauffeur - despite the fact he hasn't a licence - for a speaking engagement at the Edinburgh Festival.
The upshot is that he has to choose life at home with his sanctimonious mum and increasingly disengaged dad or chance his luck helping out Walters' barking old thesp.
Director Jeremy Brock - who wrote Mrs Brown - provides the vocal ammunition for Linney to spit out and Walters to relish in a style familiar to those who cherish her cantankerous old bat creations with Victoria Wood.
The film is particularly strong at skewering - thanks to Linney's fully realised evangelical monster - the holier-than-thou happy-clappiness of many modern churches.
Grint is fine as the youngster and his eye contact-avoiding stoicism as he comes up against Walters' unfettered irreverence is big plus.
If you can banish the slightly worrying thought that Walters played Grint's equally batty mum in the Harry Potter series, there's much to enjoy here.