While refitting my rocker boxes having re-torqued my head bolts today, one of the threads pulled. Thought i'd better remove the head in order to get a helicoil put in.
On removing the silencer, there was a suspicious rattle, and on upending it some bits fell out, which looked suspiciously like a valve guide.
Off with the head and exhaust valve out and this is what I found.
I suspect that the issue has been caused by a problem I had with timing earlier in the year, and I have overheated the guide, mixed with what looks like a slightly bent valve.
Good job I bought a spare VH last weekend I suppose!