John Rumm wrote:

For splitting the cable:
http://www.allaboutelectrics.co.uk/6...ction-box.html

That would work, however there is a fair chance there will be enough
terminal capacity on the main incomer switch on the existing CU to add
an extra cable from there (they are built to take at least 35mm^2
normally).

Or it might be worth replacing the whole top-floor disboard, given that
there are only three other circuits. An RCBO for the lighting and an
RCD group with three MCBs (2 rings and the shower) would bring things up
to 17th ed. standard and give a neater-looking job for not that much
more cost. Then if this lighting circuit feeds the shower room the need
for supplementary bonding is eliminated (provided that the main bonding
for the house is OK and there are no other circuits in this shower room).

[...] You can do it without an RCD if you can be certain that the
earth fault loop impedance is going to be low enough.

That's not an option under the 17th. All circuits feeding bath/shower
room circuits must now be 30 mA RCD protected. (I'm sure you knew that,
really...)

However the RCD eliminates the doubt (not that it hurts to
design as if it were not there)

Zs needs to be watched here, given that the feeding distribution circuit
is 16 mm^2 - twin & earth, quite possibly, although no specific cable
type was mentioned - so possibly only a 6 mm^2 incoming CPC to the
board. I'd want to take a loop reading at the board before going too
much further.

--
Andy