EricP wrote:

Nothing was apparent in the number of posts I have downloaded, or I
wouldn't have bothered to ask.

You'll find lots if you search in Google Groups. Also have a look at
http://www.niceic.org.uk/downloads/NL139supp.pdf

Briefly, you don't have to bring in an earth from outside (unless the
rising water main in in the bathroom in question). Supplementary
bonding in a bathroom is about connecting certain accessible exposed
metalwork together, so that dangerous 'touch voltages' can't arise.

The size of wire you're most likely to need is 4mm^2.

--
Andy