This is a very difficult question, generally not addressed by regulators in the past. Indeed, there is a widely-held view that it is an impossible question, that any attempt to compare the difficulty of non-cognate subjects amounts to a category error. ASCL is not necessarily of that opinion, and congratulates Ofqual on attempting it.
Difficulty, like beauty, is in the eye of the beholder. But some people, places, animals, pictures are generally thought to be more beautiful than others, and some subjects are generally thought to be more difficult.
Read the full response here.