A typical bodged diy installation Paul!
I think all of our UK members have similar experiences regarding electric shower installations.
I've seen the entire shower circuit wired in 2.5mm T&E, all the way from the board.
Looks like your RCD saved the day before a fire started.
I constantly see 45A pull cord switches failing, when you remove the switch you find burnt 6 or 10mm conductors. I don't know if the installing electricians are not tightening the terminals correctly, or if it's the switch design, all pull cord switches seem more prone to fail than wall switches.
I have a personal dislike of the British instant electric shower, flow rates are pathetic in winter, not to mention the water & electricity in the shower area aspect. If we had more unvented hot water systems like other developed countries, they might fall out of favour.
