Thanks for all the info. I adjusted the valves today and while I was there I had a quick look at all the cam lobe surfaces (or as much as I could see through the adjuster openings), and could see no pitting, and the clearances were only .0005-.0015 out, so I would say that the chain and adjuster are definitely the problem
Obviously when I do the replacement I will get proper manual instructions, but since I am sitting here at the moment I might as well ask, does replacing the adjuster require rocker cover off, head off, or barrel off? Your answer will probably govern how soon I get around to doing it!
Thanks in anticipation,