Turns out my oil leak wasn't an oil leak at all.
There was a bad connection from the radiator hose into the expansion tank.
The rubber hose was split.
Coolant was seeping out, dropping onto the swingarm, then running down and collecting at the rubber boot where it was dropping to the floor.
Enough slack in the hose to simply cut off the broken bit and re-fit.
I'm embarrassed to admit this but hope my stupidity may help someone else
:dunno
