Pecanfan wrote:

OK, I've read everything there is to know about the subject on t'interweb
and I'm significantly more confused than when I first started.

Blank canvas - new bathroom - no earth at present. The ceiling is 2.9m high
and the lighting is 12v IP44 rated with an external switch. There's nothing
else electric in the bathroom other than the extractor fan, which is double
insulated.

I have a metal bath, metal towel radiator (NOT electric), metal shower and
(obviously) metal taps. All supply and c/h pipework is copper, all boxed in
above floor level, and the mains water supply to the building is plastic.

OK, so the things to be included in the supplementary bonding a bath,
towel rail, pipework to bath and basin taps and (don't forget) the
circuit earth wire (CPC) of the circuit feeding the fan if it's within
the zones. This is the lighting circuit, most probably. (If the fan is
more than 0.6m horizontally from the edge of the bath and more than 2.3m
high then it's outside the zones.)

1. I gather I need to earth my lovely chrome towel radiator with an awful
green and yellow cable? How on earth (sorry!) do I do this without having
the earth cable dangling up my wall? Would it be sufficient to bond the
(microbore) pipework inside the boxing (approx 1ft below the towel radiator
itself) or does the actual towel radiator need to have an ugly earth clamp
stuck to it?

You can bond to the pipework "in close proximity" to the towel rail -
although a normal earth clamp ain't gonna work on microbore tube
(AFAIK). If you don't bond to the rail itself then bond to both the
flow and return pipes, as a precaution against any high resistance
joints in the plumbing.

2. I read somewhere that if the earth cable is within conduit 4mm^2 is
sufficient. Since the earth cable will run (loose) within boxing around the
room, does this count as conduit?

4mm^2 applies if there's no mechanical protection. Where there is
protection you can go down to 2.5mm^2 between
extraneous-conductive-parts (pipework, bath, towel rail) and 1mm^2 for
the bond to the CPC of the lighting circuit. This doesn't make much
difference to the visibility though, so most people just stick to 4mm^2
for everything, thus avoiding the debatable question about whether pipe
boxing counts as protection.

3. OK, so I connect everything together with earth clamps and bits of
wire - where does this ultimately need to go back to?

Nowhere, necessarily...

I've read it's NOT a good idea to take this back to the CU.

.... but it will find its way back to the CU earth terminal via the CPC
of the lighting circuit, and any other circuits feeding equipment in the
bathroom. It's wrong to say "it's NOT a good idea to take this back
to the CU" - but it's simply not necessary to do this separately.

4. If I bond everything together within the boxing, to hide unsightly earth
cables, will this be sufficient? (the boxing will have removable panels at
strategic points) i.e. exactly how close to the taps, towel radiator,
shower etc. does the earth wire need to run?

Yes, supplementary bonding can usually be done very discreetly,
especially if it's not being done as an afterthought. All connections
should be accessible, unless crimped or soldered, and labelled "safety
electrical connection - do not remove".

HTH
--
Andy