Thanks again for the replies. The 'van is a Hobby 460 UFE (2004). My Landie is a '04.
I have just measured the ball, and it is 49.75 (generally all around), which I believe is a little undersized (but I think still within tolerance). There is no paint or galavanising on the ball, and I have rubbed it with emery cloth to try and clean/rough the surface.
I do have a nose weight balance. I will check it. I do not check it everytime, but intermittently, and it always seems to be about right (but that was before the Landie and so maybe now I just need to add a bit more weight in the front of the van to bring it up to the max)...obviously depending on the hitch limit.