well got the engine/box out, found what I think is the cause of it jumping/falling out of gear.
found the rear gearbox mount has a steel plate in a sort of upside down L shape, one of the bolts has fallen out, and now I find the upper bolt hole has snapped off, so there was only one bolt holding it, instead of 3, and it was moving around.
Seeing as I have the box out I will be replacing it and when i get around to it will strip it down and rebuild it.
