If I suspect the noisy cam follower in cyclinder number four, before jumping straight to dismantling the engine, would this be a reasonable test?: Remove the corresponding spark plug; and slacken off the adjusters for both the inlet and outlet valves. The theory being that it would stop these followers from riding on the cam and stop/change the noise.
 
Thank-you all for your help, I feel like I am narrowing down on the problem.

I took another look at the adjusters today. The adjuster for number 8 valve Actually has less (not more) thread showing than all the other adjusters. The noise also seems to be coming from the back of the engine (I haven't listened with a stethoscope yet @Dippypud), so might indicate something?

This is the opposite to what you suggested @Colthebrummie (less thread rather than more), so not sure what this signifies. Do you think this is enough to start investigating the cam followers or would that be a bit hasty?
Yes, sorry. I'm getting confused in my old age. Less thread could mean a worn follower as James said. I don't think slackening off the adjuster would be a good idea.

Col
 
Re test, not sure, it will be running on 3 so it will be very rough. If you do want to check just use take the corresponding push rods out.
 
Too take the pushrods out you have to remove the rocker shaft assembly complete you maybe able to see the damage down the pushrod hole.
With rockers off worth giving valves a close inspection maybe a broken valve spring I believe they are in pairs on this engine.
 

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