for anybody interested, here is the progress. Cracks have been ground and sanded out. I need to pick up some acetone and then fill the hairline cracks, followed by building out the area that had been sanded back.
I've not yet decided which route to take. Options are
1. Epoxy filler, then get the area re-gelcoated professionally, or
2. Plastic Padding Gelcoat filler (which I think it polyester), which is white and can be sanded /polished, meaning no secondary gelcoating.