After seeing it: I'm confident the book will be better. :-(

The concertina was waved around, but the playing not even creditably faked.

It's a bit unfair to compare the representation of a concertina playing in a film with the description in a book. The book could simply say "he played a tune on his concertina" or words to that effect, without having to go into any details about which keys he pressed, whether it was an Anglo or English, etc.

Whereas in the film, an actor would have to know how to play a concertina to be able to mime the playing of a tune effectively, to the satisfaction of viewers who know how to play. The answer might be to either (a) only employ an actor who can actually play the concertina for the part; or (b) use a stunt musician to play, with regular cutting between the musician's fingers and the long shot.