Hi Richard,
I can thoroughly recommend Steve Sawford as a source of materials - I've bought a fair bit of gelcoat from him in the past and he always matches up the colour perfectly.
I recently had to do a repair to my tail end after a collision and did just as Steve Willis suggests. Thouroughly clean, then build up some new glass on the inside. In my case I wanted a bit of reinforcement for the gudgeon area so glassed in a small piece of 1.6mm FR4 laminate (the fibreglass sheet stuff they make printed circuit boards from). I later filled both gudgeon mounting holes from the outside and re-drilled. I found that the standard length bolts from Catparts were still long enough to re-fit the gudgeon.
By the way, I found it very difficult to put on more than a couple of laminations of glass sheet at a time as it sticks to everything. In then end I had a strong repair but a helluva mess inside the hull, and I had to snip off protruding bits of glass & resin with some cutters and then painted on some gelcoat to hide the worst of it and protect my hands from scratching during "sponging outs"
On the outside, it was just a case of again doing as Steve suggests and cutting away any cracked gelcoat - you'll be srprised how much the rotten stuff can extend especially if it's got water under it. Then I put three coats of gelcoat on over a weekend and left it. The next week I spent a couple of hours sanding it back smooth with finer and finer wet-n-dry, finally finishing off with T-cut for a super glossy finish.
