Bit the bullet and headed into B&Q to pick up a pair of offset aviation shears. Obtained a pair each of left cut and right cut. These are £30 well spent, they are so much better than the existing set of straight snips that I already have and cause almost no distortion to the good side of the cut.
Make reasonably short work of cutting out the engine bay side wall and head down to offer it up. A couple of tweaks here and there to get around the diagonal that supports the top of the front suspension assembly and it fits very snuggly with a sizeable overhang at the top. Drill 4mm holes and bolt it into place. It's still a little snug and fouls the earthing strap from the loom, so I will have to remove it again later when I come to trim the top properly.
Glassed in a small bracket to attach the lower portion of the mudguard in place behind the front wheel. I've prepared two brackets to secure the top but spending two long underneath the wing with the fibreglass fumes does ones head in.