Are the holes where cables penetrate the ceiling for light fittings
vapour proof or just a roughly cut hole with loads of space around
the cable? Particularly for fittings in rooms with high humidity,
kitchen, bath/shower rooms. Are any ceiling down lighters open at the
back? They ought to be fire proof anyway to slow the spread of fire
from the room to loft space.

Yes, non-fireproof downlighters in kitchen and bathroom (as was at purchase last year) but now fitted with LEDs. Didn't realise these may be contributing to moisture ingress. I'll also have a look at sealing wire holes in ceiling for normal light fittings (non-downlighter type).

Thanks