Now the fiberglassng has gone off, and early testing shows the wheel arches not to be a problem anymore, I have filled and sanded the wheel arches into a fairly pleasing shape. It makes the rear end of the car look more beefed up than the arches before.
Mixed up some GelCoat and generously applied to both sides. Once cured, I mixed up another batch of GelCoat and applied again, to give a good thick layer of gelcoat. That can be sanded, and polished failry easily.
Started the sanding of the gelcoat, this process is going take along time. Once cured the gelcoat remains a little sticky, this cloggs up sand paper very quickly. Once this has been removed, the gelcoat is tough barrier to sand. I have about 80% of the gelcoat sanded now, just a few small difficult to sand areas left to go, and one area to patch as the fiberglass is visable through the coating.