Quote - Check for any displacement mapping. Displacement effects will not show in preview but will be visible in the final render. A quick fix is to apply a very small displacement to the clothing effectively giving it a looser fit.

Ok, didn't think that was the problem to start off with, but just been doing some more testing and it looks like that is the problem after all. The clothing that had the really severe problems has folds etc. done with displacement. The amount of displacement doesn't seem to be making any difference, but the pokethrough is gone if the displacement isn't used in Firefly. So I guess the answer for me is - render in Vue! ;)

Thanks for the help!