i bought a Drayton TRV4 + lockshield set, and found the lockshield is
pretty much the same valve body as the TRV but with a plastic head
that acts as a manual valve, operating the valve plunger as you turn
it.

This surprised me - shouldn't the lockshield be set using a tool, with
a cap that doesn't operate the valve? This one seems to make it too
easy for someone to open/close the valve and unbalance the system.

I think I'll go and buy a more usual lockshield to use instead.