I wouldn't use Nitromorse on plastic mate as it will melt/ blister the plastic. personally i would just scrub the plastic down with either scotch pad or wet and dry (prob 240-400 grit will be good) to key the plastic and remove any flaky paint, wipe down with some window lean and prime with a plastic primer which you can b=get from Halfords, it is yellow tho so you will most likely need to either do loads of thin coats of your blue or spray over the plastic primer with a normal gray primer before laying your blue.

Nice one, ta slice

What about re-painting? im assuming i will need some sort of flexable primer and a laquer to finish?, unless i can get it in two-pack

Yes and yes, but be careful to never spill fuel on the tank because the laquers dont like the stuff!...I assume you're doing it with aerosols

well that's going to happen at some point, even if its just a drop or so from the filler, i have a spray gun i think it would be best to sell the black fenders that i have now and buy some blue plastic ones, atleast then all the chips and defects that will inevitably occur will still show through as being blue instead of the original black.