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I have recently moved house and there were two yellow and green earth cables hanging loose under the kitchen sink. I have since connected one cable to an earthing clamp just above the stopcock so that the mains water pipe is earthed. However I was wondering if it was still necessary to earth the hotwater pipe from the condensing boiler to the kitchen sink.

It is all down to the version of the wiring regulations that the house is wired to.

It has been realised with RCD protection any earth leakage will likely trip the RCD and when assessing the risk, there is a higher risk from high current through the body due to good earth, than low current due to bad earth, so much of the earthing is no longer required when wired to BS7671:2008 or latter, but BS7671:2004 still needs the earth wire as all circuits not RCD protected.

There is some debate, with plastic pipes no earth required, but with metal pipes a fault in one room, like dog knocked over a standard lamp which has a broken bulb resting on radiator, could be transmitted to another room, so touching radiator in another room could give you a shock.

However in real terms there is normally enough of a natural earth to cause 30 mA to flow so the RCD does trip.

Remember a RCD does not stop you getting a shock, neither does it limit it to 30 mA, what it does do is limit it to 40 mS but that current could be 100 amp.