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?