No longer smoking!
At last I took it for test run today - and all is fine. So, what did I find/fix?
The loose breather cork - replaced
I DID find a leak in the body of the oil pump - sent it to J Brown Services on Hayling Island for refurbishment.
Fitted new rings - just in case.
Checked and cleaned the oil pressure relief valve.
I've done a lot of other stuff too, but none of it related to the smokey engine - such as magneto off to clean oil out, dismantle clutch to get it working smoothly, replace studs in the rocker cover, make and fit new head steady brackets, fix wiring issues. And replace the magneto pinion which broke after I followed the instructions in the Haynes manual - you shouldn't hit fibre pinions with anything!!!
So now it's all sorted, running nicely and ready for the sun to start shining - and Boris to bless club runs again!
Many thanks to all who gave me such valuable help.