Hi James,
Great memory, that would be me. Still in Ireland but not racing unfortunately.
I am using Shell Advance Racing M, ratio 20:1. Not seeing a pattern on the crown. The surface colouring is consistent across all the piston, a glossy purply/brown colour. The petrol used was fresh. Possibly I am rich, the main jet used is what I have used in winter months. However, the plug was not oily, looked a lot closer to lean than it did to rich.
Could not running a cold enough plug be the reason?