Paul
Its going to be a process of elimination and going backwards to come forwards. My suggestion is to:-
01 - Switch all your mcb's OFF to every circuit on the board.
02 - Switch all your plugs on your sockets back on.
03 - Switch the RCD to on
04 - One at a time turn each mcb back on
05 - At the first RCD trip OFF, turn the mcb off that appeared to cause the RCD to trip.
06 - Carry on turning on the balance of the mcb's
From your description above I would assume that something on socket circuit(s) is causing the RCD to trip.
01 - turn off that circuits mcb & Switch back off/unplug all devices on the circuit causing the failure.
02 - Switch on the RCD
03 - Switch on the mcb causing the failure. If the RCD fails without any appliances on then you have a circuit failure.
04 - Proceed turning on one at a time your appliances.
05 - If an appliance being turned on trips the RCD then you have found a faulty appliance.
I would assume that whoever installed the new board with RCD would have carried out an insulation resistance test, therefore making a circuit wiring issue a slim cause.
Matt
January 2009