Karl,
As Paul stated in his post, here in the UK we have been prohibited from using the water pipe as the sole means of earthing an installation since 1966.
We are however required to bond all metal pipework in an installation to the main earthing terminal, regardless of whether the supply from the street is plastic or metal.
The requirements for this equipotential bonding have become increasingly stringent over the last 20 years or so. The minimum size for main equipotential bonding conductors to water, gas, heating, structural steel etc, is now 10mm for small installations up to 100A supply, but the local PoCo may require a larger size.
Supplementary equipotential bonding is required in areas of increased shock risk. In a residential situation this is bath/shower rooms, where bonding is required between the earthing terminals of electrical equipment & extraneous-coductive-parts (pipes) installed in zones 1,2 & 3.