Finally found time to start snooping around the rear cylinder to see what was creating the (presumably) heat-related noise.
The head looks OK with nothing obviously broken or bent yet; it's a '56 head, with the later o-ring manifold. Would have liked '52 heads, but am happy not to have the hassle of the plumber's nut intake.
Scoring in the cylinder looks a little ominous. Wonder if the piston has similar marking. Pete's coming to have a look-see this weekend as I have no idea what I'm doing!