Hi Terry

In the part of the UK where I live , the Electric Co. (used to be YEB ,God only knows what it is now) banned using the water pipes as an Earth in the 60's (as per 13th edition IEE regs) So although they are used for EQP bonding,they cannot be used as an Earth.

The Electrical Research Association part of the IEE has carried out tests in the UK on the electrical conductivity of water in plastic pipes , and they have found that UK water is quite a poor conductor (due to impurities and suchlike).
They quote for 1 meter plastic pipe..
15mm diameter, tap water 115kohm
20mm diameter, tap water 65kohm
(obviously this varies slightly depending on where you are in the UK).

Generally the IEE believe that plastic piping is safer.

Rob