Thanks guys
I've seen the heat gun method applied to faded black soft plastics like ABS and seems to work but I've also tried compounds that do much the same with less risk.
For black plastics., I swear by Autoglym Bumper Care - the green snotty stuff, not permanent I know but does the business, I recently used it on a faded plastic patio table which originally was a dark plum colour and it had faded to pink, used it on that with great results, but this same snotty stuff has no affect at all on the red plastic buckle catch, so maybe a different sort of plastic altogether.
Anyway I will get a replacement first before getting the flame thrower out, I think the difficulty will be to apply the heat just to the red bit without melting the black surround which is possibly a bit thinner and softer material.