Well. I got the idle sorted. Took the bike for a run in the forest, and after a little while, the engine started to knock suddenly! Trailered it home, and started pulling bits off. Finally took the head off and found this.
IMG20181020133325.jpg
A frigging steel spring washer got into the combustion chamber, and bounced around for a bit, before getting jammed on top of the piston, and bashing into the head.
Luckily the valves and seats are ok. Wonder whats going to happen next????