While apparently travelling south on the Grand Union from Braunston to Stoke Bruerne toward the end of the film the boat traverses Blisworth tunnel, but is actually travelling the wrong way, from the south portal to the north; the towpath appears on the right when, travelling south it should be on the left.
Several times the boat is going from Braunston to Blisworth (heading south), but the lock in use is Stoke Bruerne top lock, which is south of Blisworth and going the wrong way (locking up which is northbound).
Mr Smith is told to take his butty to Nurser's to be converted to diesel. Whilst this happened in the early days, it would certainly not be the case in the 1940's. The modernized 'Sunny Valley' is clearly not the same boat 'upgraded'. Mr Smith would have been given an new motor boat and told to use his old boat as the butty.