I realise that, but wondered about the warning that comes with RCD
sockets. It suggests (to me) they may be damaged by testing?

All RCDs I've seen come with a warning to be removed when insulation testing
(presumably they either don't like the high voltage, or would affect the
result). This is usually in the form of a sticky label that you're supposed
to put by the consumer unit so that the sparky knows they should be looking
for one to disconnect. I can't see any reason to disconnect an RCD socket
for an earth loop impedence test unless you are intending to use that very
socket for the test.

Christian.