Gear Box was stripped down, without removing the engine and rebuilt in place.
The box had been more or less new, so most likely one of the factors causing the problem was the lack of bedding in before the session on the Dyno.
Also during the rebuild I noted that the bores in the casing were the bearings sit had started to wear oval, allowing the shafts to spread apart slightly.
Anyway it has now been rebuilt with some used gears & has completed 200miles without any problems.
However I am looking into ways to strengthen the gearbox, or the possibility of a different box...
It only took a few days to get the box repaired & then the car was out and about again. Seen here on the way to a photo shoot for PPC.