js.b1 wrote:

Mapei Primer G is a good water resilent primer,

Ah ha! I actually have a few bottles of that in the shed left over from
the tiling. Of course, I've installed the mirror frame in the cavity now
so it won't be quite as easy to apply, but if the consensus is that I
need something that's probably what I'll use.

If you can, consider getting a quiet humidistat controlled fan in the
bathroom. Whilst some condensation will still occur since it can't
keep pace, it will reduce it somewhat.

Yep - got one, but not wired up yet. (Actually, it's not humidistat
controlled, it's on an overrun timer off a flow switch in the shower
supply, but same idea.) Hopefully once that's in action there'll be less
condensation about.

The lights incidentally will provide some heating which may mitigate
condensation (just a thought).

Indeed, the lights (fluorescent tubes, so presumably not *hot*) are
supposed to be warm enough to stop the mirror fogging.

Pete