Simon,
I hope you have found your problem - although looking at the photos I'm not that sure that it could have worn enough to cause a knock?? could have been some debris in there that caused the damage - hard to tell, but look on the bright side - at least you have found something!! Check the bottoms of the bores on the block and study that rocker assembly for damage - there just could be a small piece missing that has appeared in that bore (that would be the most likely place for it to go if it were the rocker)
GUMBALL UK
P.S. you really should put the cap back on the Hylomar - that stuff gets everywhere!!
