Are you saying that you have a separate sub -consumer unit in your garage and even when this is switched off, the rcd at the main consumer unit in the house is still tripping?
Note you say your showers and sockets are on same rcd. A shower must be protected by it's own separate 4OA/30mA RCBO (or else RCD with an MCB) - not lumped together with sockets. Also if there is more than one electric shower they need to be interlocked to avoid overloading the supply (although that's not relevant to your present problem).
J
October 2008