I agree with leaving the hot water system off for a few days.

My mother in law had a earth leakage trip problem a few years ago. turned out to be an ants next on the back of a power point. all would be well for a couple of days until they would rebuild their nest then the voltage would pop the nest and trip the RCD, then all would be well again for a few days, this wnt on for a month or so until i found the problem and removed the ants nest and replaced the power point.

so as you can see it could be anything causing it but in your case i would think the hot water system if it is on the same circuit as the power points.