So decided to have another go at this.
Working in the garage with a torch let me see in the gubbins easier than outside in sunshine as when I last tried this.
Removed brake lever and swung out of the way
Moved breather pipes aside.
Removed bolt from crankcase .... didn't look too bad, but cleaned everything up and put it back together.
Hardest part was removing and replacing brake lever to master cylinder pivot split pin.
Put a wee bit of paint on the bolt so I can see if it moves.
Haven't actually tested it yet as it is pouring with rain in Edinburgh and didn't want to fire it up for two minutes idling.
I will report back later if you can bear the suspense