Hi Robin,
I see you are making swift progress on dismantling your roadster.
I had the same problem with my Sabre body tub and this is how I approached it.
I needed to clean up the area where the original builder had mounted the pedal box and brake servo. It was a bit of a mess from the original builders holes plus some holes I made trial fitting other types of brake servos. First sand off all the original paint and primer. I chamfered the holes and screwed a piece of MDF covered in packing tape behind the holes. Brown packing tape is great because the GRP doesn't stick to it. The screws hold the MDF in place and can be easily filled later.
Brake M/C mounting area repair by
Sabrebuilder, on Flickr
I then filled the holes roughly with Isopon P40 fibre filler. Difficult to see in this photo as it's clear.
Brake M/C mounting area repair by
Sabrebuilder, on Flickr
Then I sanded down and filled with Isopon P38 and then sand flat and smooth.
Brake M/C mounting area repair by
Sabrebuilder, on Flickr
Finally, I mixed up some GRP resin and put two layers of glass fibre matting and tissue on the inside to leave a smooth surface for mounting the pedal box.
Brake M/C mounting area repair by
Sabrebuilder, on Flickr
I primed and then filled some small indents with a fine surface filler. Then sanded flat with wet and dry and then another coat of primer.
Brake M/C mounting area repair by
Sabrebuilder, on Flickr
I learnt all this from this forum so thanks to the bodywork experts!!
Have fun..........
.....peter