I extracted the following from a forum on the IEE site. It was in reply to a question such as you have posed, viz., how to test a suspect RCD. I Hope it helps!
"With circuit isolated, disconnect any current using equipment from circuit.
Disconnect outgoing cables from RCD and test...
Your problem could also be why my main RCD tripped when I was repairing a socket, which I thought was completely isolated. However, I believe the mcb controlling the ring main only isolates the live conductor, leaving the neutral connected to the neutral busbar. The main RCD tripped whenever...
I need two extra fuseways for radial circuits supplying two storage heaters. I don't have any spare on my exising consumer unit. Can I add a smaller (say 3-way) consumer unit to the existing one? If so, do I need to get the electricity company to connect an additional set of 'meter tails'?
A good idea! But if the box wasn't proud enough, I suppose you could screw a couple of sharpened screws (points where the heads were) into the box lugs and use them to mark the plasterboard?