Hmm I agree about the bearings, was just throwing ideas about.

Is there an easy way to see if cams over worn? ie can I just take a lifter out and feel the lobe or do I have to remove the cam as a whole?

Ordered a new inlet valve today so I won't be trying to lap the old one in to what appears to be a nice unworn new seat. The other three inlet valves are like brand new without even any deposits on the stem or under side.

Going back to where I briefly mentioned the springs....another theory maybe; the head has been unused for about two years. Is it possible that with both the inlet and exhaust valves on cyl#1 been stuck open about <5mm for that length of time the springs have compressed to a smaller size permanently. ie they've been weakened and are now not returning the valves to their normal closed position. This would account for normal operation at tickover when the spring doesn't have to work so fast, then when revved it can't keep up creating a gap between the lifter and the cam lobe too large for oil to gap so it's audible as a rattle when the slacks taken up again on the next lift phase?

I've never read or been told the effects of weak springs but i'd expect that sort of behavour??

Here's to clutching at straws!