I have very vague memories of a children's/YA book that I borrowed from a library in England some time in the early 2000s. It was written in English and I don't think it was translated. I'm not sure if it was plain historical fiction or if there were fantasy elements, but I'd lean towards the former. It might be a standalone novel or part of a series, but certainly I didn't read any further if it was part of a series.
The viewpoint character was a boy who became assistant (squire?) to a fat knight named Sir John. There was also another boy serving in the same role; I think one of the boys was called Alan, but I'm not sure which one. Towards the end of the story, the second boy turns out to be secretly a girl in disguise (maybe there's some kind of bathroom scene where this is revealed?), and there's some kind of joke on "his"/her name: her real name is a feminine version of the name she'd assumed while posing as a boy.
The cover of the book was I think red or yellow in colour, and featured a cartoon-style drawing, showing Sir John as ridiculously fat. The book itself was quite thick, definitely a novel and not a novella or short story.
I know this isn't much to go on, and I can't really remember anything about the plot except the boy/girl reveal. Hopefully it'll be enough for someone with better memory or search skills than I to find the book.