Suplementry bonding, should be made for circuits supplying class I and II equipment to accesisible conductive parts within in the room.
Metal pipes supplying, water, gas, waste, central heating, air con, accessible metal structures etc...
It may be installed outside the room but close to the point of entry to the room.
It could also be omitted if certain conditions are met with acordance to 701.415.2 and 411.3.2 of BS7671(2008)
I did some more diggng, and it looks as though, under the 17th edition, if you ensure that all the equipment in the bathroom is covered by a RCD then supplimentary bonding is no longer a requirement.
In my case, I will be installing an extraction fan (in the loft) and lights (inc a mirror light), so I can use a RCBO, as the light cct is not covered by the RCB in the consumer unit. I don't intend to have any other electrical items in the bathroom (towel rail, etc).
The RCBO is reasonably costly (£20) but saves a lot of mucking around with cabling. :)
PGN As you say suplementary earth bonding in the bathroom is not required if other conditions apply but is still allowed.
As the work is in a special location (bathroom) this is covered by part p of the building regulations and you should go through the local building control or get someone who is part p registered. It will save you a lot of problems in the long run particularly when you come to sell the house.
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