I massively appreciate the suggestions, most of which will almost certainly come into play sooner than later.....but thankfully I found, isolated, and removed the knocking cause. Within the clutch;
It was (see pic attached) caused by some excessive protrusions from the clutch spring studs behind the clutch centre. In the pic I had already filed them down by around 0.035" each! These protrusions were at times lining up with the drum bolt heads, as there was clearance I had assumed they would not contact.. This would have been the case, but I sadly didn't consider the clutch drum wobble, which evidently was enough for them to whack together. (In my defense, I had seen this as a potential cause way way back, and I worked out the clearance between the two studs and deemed them to be too far apart to make contact). My emotions are a mix of feeling somewhat sheepish (should it have been obvoious?
) and great joy
A big takeaway for me is how tricky isolating a noise can be around aluminium casings.....the noise just seems to be everywhere.....like magpies calling....you just don't know where they are!