I took out my dodgy seat yesterday and found the fibreglass shell was cracked, as suspected.

I got £7.99 worth of fibreglass kit from Halfords and I hope it will be OK, given it was my first piece of fibreglassing. I did 2 full strips about 3 inches wide over the top edge and down both sides of the cracked part, then another bit doubled over fixed over the top rolled edge.
Rescued attachment 2005-11-20 ST seat repairs.jpg

I had to chop a two inch section out the center of my nose cone to make it fit. I put a few strips of glass matt on the inside but it didn't last long. My fantastic uncle leaned on it when looking at the engine and it burst. Since then I have redone the glass but used a foam triangle shape to form the glass.
It makes it much stronger.
I'll try and put a pick up to show you what I mean.

I have a pair of Stuart taylor fibreglass seats that I had covered in leather, the exact same thing happened to me .The first time i leant on it whilst getting out on a ramp onto my trailer, split the righthand side, I repaired it in much the same way and it was fine , then later I leant on the seat back whilst reaching behind the passenger seat and the other side cracked, i repaired this the same and the seat has been fine and now feels much more sturdy, as it tended to flex before.
Did you remeber to drill the end of the crack to prevent it spreading further though?
Paul.

I didn't drill through the end of the crack, but it already went right to the angle of the seat (between the side and the base) and I folded the new glass mat under the base as well as down the side, so touch wood it will be OK.

To repair glass fibre is always a bit of a pain, especialy when the moulded part does not appear to have enough build up in the first place, (if it had it wouldn't crack) . In a case like the one described I would be inclined to do the repair with epoxy adhesive, which is far stronger. I would also lay up a couple of layers all over the underside of the seat moulding, (rough side), to give added strength. It should be noted that you cannot use ordinary glass fibre with Epoxy, as it will not disolve the emulsion binder, you have to use glass with a powder binder. CFS Fibreglass Suppliers do mail order for all this stuff, VERY much cheaper than buying car repair kits from Halfords.