Something I never realised until today - bear with me!
Gresham (Malcolm McDowell) flies an SE5a throughout the movie, yet is described as having a Nieuport that he "absolutely swears by." This is quite often listed as a mistake by various movie websites, but here's a thought....
Now, it's a little-known fact that "76 Squadron" is very loosely based on 56 Squadron, and that
Albert Ball was an early member of said unit. It's also known that he flew a Nieuport on lone patrols.
So, by the rationale of the film's script/dialogue, we can assume that Gresham is, in some ways, based on Ball. We never see him fly a Nieuport throughout the film because, obviously, he's never seen flying a solo patrol. (Well, that and they might not have had the budget for those scenes!!)
So, what could be described as a "mistake" could actually be quite accurate, since we never see the main character flying his beloved Nieuport...??