you need to get an earth loop impedence test done fast,do not touch any water taps you do not say where you live,i bet its down south somewhere get a
pir done the rcd trips if leak goes above 30 mill amps
it could be a fault in the mains earth to house,has any building work been next door ?or any comp digging up road? as has been mentioned,check the earth bonding,but if you have the 17 edit consumer unit fitted,this is not an issue
gts, January 2013