CB wrote:

In my youth I seem to remember high quality plugs with a small grub
screw on the plastic spacer for the purpose - haven't seen one like that
for a good while however.

The metal Maplin ones have a grub screw

http://www.maplin.co.uk/p/high-quali...oax-plug-fd85g

According to the reviews, even the cheapo plastic ones have a screw, but
it doesn't work too well ...

http://www.maplin.co.uk/p/plastic-coax-plug-yw08j