After the dust had settled I went back and had another look at the roll bars. In fairness, the holes weren't too far out but they did need a little bit of work. I've narrowed the problems I had down to two reasons.
1. The first set of bars fitted fine but for the second, it wasn't obvious which way the holes needed enlarging when they wouldn't initially fit. They appeared to be tight against the bodywork but in reality there were simply leaning forwards because of the third leg hanging into the cockpit. If I'd had the chassis level I could have checked the vertical.
2. I hadn't taken account of the flex required in the third leg, I allowed for the main legs but cut the rear hole to fit the bars in their unflexed state. Once flexed to fit the mounting points, the hole had been enlarged in the wrong direction.
I didn't take pictures of every step in the repair but with the few I have and a desription, hopefully you'll be able to follow what I did.
The first thing I did was to tape a piece of 1mm rubber mat around the roll bar where it passed through the body. I then cut a curve into a square of grp matting and glassed it in on the underside of the rear deck, up to the rubber mat wrapped around the roll bar. The rubber mat was to protect the roll bar and once removed, to give me some clearance to remove the roll bar afterwards.
When it had set, I filled the gap I'd created with Isopon p40 filler and then when that had set, removed the rubber mat and then the roll bar itself leaving a new fillet of grp filling the space which I had previously over enlarged.
Now because I had filled up to the roll bar itself, the repair was too much and needed cutting back to give the required clearance of 5mm all the way round.
Prior to making this repair I had cut two doughnuts out of a 1mm rubber sheet, both with the same outside diameter but one which had the same inside diameter as the roll bar and the second with the diameter that the hole needed to be, including the 5mm clearance. With the roll bar in position I placed the first doughnut around the roll bar and marked the outside circumference. With the roll bar removed, I placed the second doughnut into position, carefully lining up the outside circumference with the marks made for the previous doughnut. I now had a reference point to enlarge the hole, along with the repair, to give the neccessary clearance.
Once the hole had been enlarged to match the inside circumference of the second doughnut, the filler was also cut back on it's surface ready for gel coat to be applied.
Once the gel had gone off, it was rubbed down level to the bodywork and repolished before the roll bars were refitted to see if it had all been a success...
There may still be a little fettling required but it looks good to me and with the grommet fitted, nobody would ever know...!
A couple of gratuitous shots to show how it's looking.