Pleased to report that my ailing GRiSO is now back in action.
Pinion nut had come loose and the locking washer knackered the final drive bearings,luckily no damage to the bevels.
Good job I caught it in time..... Phew!
All is now well and have just had a couple of hours out and about on a very nice Autumn morning.
Many thanks to all who have given me help and advice on this one especially Pete Roper who is a font of knowledge...thanks Pete.