Well, I decided I'd had enough of trying to repair the gelcoat, so decided to chance my luck with a gloss black Halfords rattle-can....
So far, it appears to be a fairly respectable match, and even if it's not perfect, it's certainly MASSIVELY better than the finish I was getting with gelcoat repairs (gelcoat colour wasn't consistent).
The pic below, was taken after I machine cut the surface back, and hadn't been polished, so it's slightly matt.
