FWIW the military advisers to the film included two people quite well known to several ARRSE Denizens. The military adivrs were David Kenyon (whose Phd Thesis was on cal;varty on the Western front) and Andy Robertshaw curator of the RLC Museum.Auld Yin
I have a copy of Michael Morpurgo's book if you want to take a peek.
From the Mail link :
Is it suitable for Arrse ? It is written from the horse's perspective. You might ask one of our tame Battlefield Guides who has knowledge of the last use of cavalry in 1914 to give a view.