Now, you can always make a small hole bigger but making a big hole smaller poses a few issues. I guess there a few ways I could've tackled this, I understand there is a filler cap with a larger flange which would've covered the mistake but it looked a bit ugly to me and cost about £80. I could've made a collar to fit inside the hole and reduce the inside diameter but how would I fix it in place. An additional challenge faced me in that when I measured the fuel filler in the hole, the fixing bolt holes were directly in line with any repair I attempted to make meaning that whatever solution I chose it had to be strong and solidly fixed to the body. In the end I chose to re-affix the part I had drilled out which was still lodged inside the holesaw after I abandoned it in digust onto the workbench. Luckily there were enough marks left on it from the original masking tape that I could use as a guide to position the piece precisely where it had come from. I then used additional masking tape and drew some guideline upon it to ensure I had it positioned correctly before suspending the part in it's orginal position using a couple of battens and using a fibreglass kit to place a strip either side and glass the piece back into position from the underside.
Once the initial repair was dry, I removed the batten, placed a strip of fibreglass in the middle and then two extra pieces which covered the whole area.
I then filled the hole which was left by the original holesaw cut with some fibreglass filler before cutting back with the trusty Dremmel and filling with gelcoat and sanding back with wet n dry.
I was quite pleased with the end result and whilst perhaps not as good as it could have been, it was far better than it needed to be considering it would all be covered by the flange on the fuel filler anyway. The hole was drilled once more, this time with the correct size holesaw and finally, as a belt and braces job, I made a plate from 2mm aluminium to fit on the underside of the body for the fixing bolts to pass through and strengthen the whole thing up, effectively clamping the repair between the plate and the filler itself. I will need to get some longer bolts to pass through the extra thickness of the body.
Whilst I would never have chosen to follow this path, I did quite enjoy myself and learnt a little about fibreglassing along the way.... can you tell I'm still trying to put a brave face on....sob, sob !