About to install a "mirror light" over the washbasin in the bathroom.
It is a mirror with a metal box on the back containing 2 compact
fluorescent G23 lights that shine through un-mirrored areas of the
mirror glass. It is rated IP44. There is an earth connection within
the box. The light is switched by a short pull cord.

The instructions that come with the light say "This mirror should be
fitted to a lighting supply protected by a 5 amp fuse or equivalent
circuit breaker. If fitting in a bathroom the mirror should be
connected to a supply protected by a 30mA RCD. "

The instructions also contain a diagram showing the bathroom zones,
with a Zone 2 being defined for the area within a 60cm radius of the
basin tap. Zone 1 is defined as requiring IP45 or better - with mains
voltage fittings requiring a separate circuit and a 30Ma RCD, with
Zone 2 requiring IP44 or better.

The instructions however make no mention of supplementary bonding...
...................

The elctrical installation has PME earthing. There a a "split"
Consumer Unit, with only power sockets that serve the outside of the
house protected.

All pipework in the bathroom is plastic.
The IP44 ceiling light in the bathroom is in Zone3 (2.7 meter ceiling)
as is the bathroom extractor fan. These are both on the same curcuit,
with a MK 3 pole lockable isolator sitting between them. The light
switch and the isolator are outside the room. The fan is about to
bemade separately switchable via a pull-cord.

My question is: Is protecting the "mirror light" with a RCD a
'sensible'
thing to do in my situation?

I don't really want to have the ceiling light protected, so it would
mean either a separate curcuit from the protected side of the CU (if
that is allowed) for the "mirror light" - along with supplementary
bonding to the ceiling light, or putting in some form of RCD spur
(TLC's Order Codes CM 4904 / BR H92 ) ...and finding somewhere to put
it and regularly test it !) specifically for the mirror light.

Ray

The instructions however make no mention of supplementary bonding...

It will need supplementary bonding as it has an earth terminal. However,
with plastic pipework, there might not be another fitting in the room that
requires it. You shouldn't supplementary bond metal baths or radiators fed
by plastic pipe.

The instructions also contain a diagram showing the bathroom zones,
with a Zone 2 being defined for the area within a 60cm radius of the
basin tap. Zone 1 is defined as requiring IP45 or better - with mains
voltage fittings requiring a separate circuit and a 30Ma RCD, with
Zone 2 requiring IP44 or better.

That sounds all wrong. Zoning is in relation to the shower tray or bath tub.

bonding to the ceiling light, or putting in some form of RCD spur
(TLC's Order Codes CM 4904 / BR H92 ) ...and finding somewhere to put
it and regularly test it !) specifically for the mirror light.

This is what you should do. The manufacturer's instructions say it should
have an RCD to be suitable for bathroom use, so this overrides any general
requirement in the regulations.

Whilst you are there, you may wish to rejig your consumer unit so that all
socket circuits (except the fridge/freezer) are RCD protected. This greatly
improves the safety of the installation.

Christian.