Well, a few of you might be interested, I have found a newish rocker arm to replace the damaged one in my 500LS.
A well known german parts supplier wants 71.90 euros for one! I got a perfectly useable one off the 'bay for 8 pounds. I discovered a bloke that seems to dismantle 550 Honda's, and the rocker arm is the same, probably more durable.
Engine manufacturers buy in components like these, so there is a chance they are of oriental origin anyway.
Lets hope that wretched tap tap tap is a thing of the past, I'll put it back together at the weekend and see if it is.