It looks like levelling up is the issue - both main characters were about to level, so the next gain in XP (combat or adventure award) caused the crash.
I fixed this problem by spending all saved skill/attribute points for the two main characters. I didn't need to dismiss the two henchmen in order to get past the problem.
Obviously this could still be a problem later on, but at least I know of a potential work-around.
Cheers,
Z
