Seal concrete block wall?

I think that would be wise. I would probably use bitumastic paint, but it might be subject to condensation. Perhaps a skim of plaster over the cut block, and several coats of emulsion paint?

PVA is not suitable, because the adhesive "re-activates" when wet. SBR is waterproof but too expensive by virtue of its 5L unit quantity.

Mapei Primer G is a good water resilent primer, priced at =A34 from Screwfix - basically not much more expensive than PVA. Mapei Primer S is a fully waterproof version, but you're not planning on a swimming pool I assume :-)

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.

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

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.

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.

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

Pete