A supplementary question. 'Additional' RCDs come with a warning about
disconnecting before performing earth testing. Does this apply to a CU
one, and if so what is the method of testing such circuits?

Either bypass it or use a tester designed for the purpose.

The reason to bypass them is that the standard earth loop imdedence test
passes 20A from live to earth, which given that most RCDs are rated at 30mA
might just cause them to trip!

Christian.