The problem with this rad appears to centre on the shallowness of the
female screw thread on the rad (I will measure that and post here when
its next off). As the valve tail is being screwed in it tears off the
PTFE tape by pushing it off to the outside of the rad.

I'm now thinking that either LS-X or liquid PTFE (which is terrible
for getting a screw fitting so it can be subsequently undone) may be
the only answer.
Tom