Well, the trimming went well. The Bosch mini angle grinder is great for cutting nice straight lines, and I used a Dremel with a sanding drum to shape the curves.

The whole Bosch Professional 10.8V range is excellent, the tools are just so much lighter and more wieldy than the 18V jobs, and they are very nearly as powerful. Highly recommended.
So having cut out the bits to go round the seatbelt mountings and the frame rail. I offered the wing up. The bit I was worried about was whether the curvature of the wing would match the curvature on the car, but all was ok and even propped into position with a couple of bits of wood there's still only a 20 thou gap, which will easily close up properly when I put the fasteners in.

I'm also very pleased with the colour match of the gel coat, I think the wings will polish up very nicely and I can avoid having to get them sprayed.