John Rumm wrote:

ok understood. The rear access to the crankcase is easy enough to
assess. The front of it is obscured by the housing at the moment until I
can work out how to take the final output and clutch off.

Fill the crankcase with parafin and see if the level drops over night.

Typical FOD is from leaving the air filter off but I have seen screws
holding the throttle butterfly come off, the middle electrode or ceramic
from the plug being lost and bits off the ring ends.

AJH