my bet would be the little 50 watt hot water system is on the same circuit as the rest of the garage which did not have an RCD when it was installed but over the years an RCD was installed without taking the hotwater system in to consideration thus it remaind on the same circuit with the rest of the garage.
i could be wrong but i have seen this happen once before.
I agree in getting a sparky to check it all out.
You are on the money Beaky! The wiring to the garage (including the little hot water system) goes back to the main house power board on one circuit. The sparky has made the necessary modifications. The hot water system is 10 years old and is slightly leaking so I will replace it.
