I then had original-looking paper but still the dreaded mould:
It was a bit difficult to make out some of the features due to the spread of fungus and I didn't really want to add any pastel for fear of masking the original artist's style. So very gingerly, I set about with a stiff bristle paint brush ( artist's, that is, not decorator's!) and worked away at the mould.
Here you can see where I've tackled it and which bit is still infected:
I simply redistributed some of the pastel and it's almost as good as new. Ta da!
There are still some cracks visible, but I don't think that's a bad thing and I even managed to preserve the artist's signature.