Much investigation of the intermittent RCD trips this afternoon (I'm shattered after spending all afternoon unplugging things and plugging them in again.
One thing we discovered, which may well be a red herring, was that my APC UPS, when switched off, with no power being fed to it, exhibits a Neutral/Earth near short circuit on its input. (0.4 Mohm)
Is this normal? It cannot be permanent, else the RCD would trip the moment the UPS input was powered on. Does the UPS perhaps strap the earth and neutral together when there is no input power to provide a false earth to the hardware being protected? Or is it just faulty?
(Looks like the trips are, as expected, N/E leakage on the external feed to the Klargester and workshop. [Forsees large bill for replacing the workshop supply. Sigh.])