The head was leaking into the studs but not right out which is why I didn't spot it. Looking at the cylinder head in the photo, it looks to me like it has been like this for a while.
Also, I run with a RamAir filter straight on to the plastic bellmouth of the PHBH and out through a hole in the panel. The bellmouth had vibrated loose (threads knackered) with the Ram Air still attached. Whilst it hadn't actually fallen off, I suspect it was drawing in more air than normal.
This is actually the first holed piston I have ever had (I've had plenty of seizures on cast iron barrels

) so mustn't grumble. Bits of aluminium get everywhere, crank, reeds, bearings, seals.
I'm planning on flushing out the bearings and replacing the seals. Is that reasonable?