In my limited experience it is very difficult to get a good bond with old fibreglass using fresh polyester resin and glass fibre. There are so many new products available now such as carbon fibre or kevlar cloths and epoxy resins that I think there must be better material to use. 

Many thanks. You suggest using an epoxy which I, too, was feeling would work better. Yes, it is possible to reach inside and you are right that the outside could be secured to maintain the shape which will make everything easier.  With the repair started in the inside and allowed to harden first to provide most of the strength the outside should not need too much filler I am hoping. I had not thought of treating the breaks in stages - that's a good suggestion. There is plenty of time as I need to do work on the steel sidecar chassis - another story!