Right I had a quick (5 hours) look at this last night. I've figured out that the live cables in the isolation box had burnt through the earth cable thus causing the tripping out problem. I couldn't work out if this was a new problem or one that was existing before I changed the shower as I had...
I have a strange situation where a newly fitted 10.5W shower keeps tripping the RCD switches, it statred last night 4 weeks after it was first used. I spoke to an electrician mate who works next door and he said that the shower is too big for the circuit (makes sense) but why would it take 4...