Thank's very much folks.
Getting your info has given me confidence to investigate further, make a more reasoned diagnosis and carry out repair /change the seals if necessary.
Will report back on my progress. Probably asking if anyone knows how to re-assemble it. :-)
Great, thanks for the quick answer and pointer.
Don't seem to have lost water from the rad. Which makes me think that it could be river water after all. I have taken my bike for a couple of healthy swims recently.
Hello knowledgeable ones. If the gear oil is coming out grey and milky after about 6 hours use is that normal.
Someone told me it could be river water that's been sucked in, or head gasket or water pump seals.
I've come late to trials - the riding is tricky but the maintenance seems a minefield.