Err, I thought that the 16th edition regs required EVERYTHING in a
bathroom of a metallic nature to be earthed?

Some choice quotes from:

http://www.iee.org/Publish/WireRegs/...asticPipes.pdf

Metal radiators supplied by plastic pipes should
not be supplementary bonded. It is not safer to
supplementary bond them, it is safer not to.

For entirely plastic pipework, it recommends to bond:

Earth terminals of protective conductors of class I and
class II equipment and accessible exposed-conductive parts
of the building structure.

It continues:

Bonding of metal taps, metal radiators or metal
baths is not required unless the bath is connected
to the metallic building structure.

Hope this helps.

Christian.