Strange damp problem

[mrrusty]

Could be - but no issues anywhere else and it seems a massive coincidence that the carpet has only gone bad under the unit?

It's not coincidence. It's what happens. Dew point of "room air" is quite high - 12-13 degrees. Condensation inside a house is very common on the coldest spots. Where there is ventilation you end up with a balance - sometimes vapour condenses, other times the room air is warm enough to evaporate the condensation back in to the air. The circulation ensures that fresh unsaturated room air is able to evaporate the surface condensation.

BUT where there is no air circulation/ventilation, the still air close to the dampness becomes saturated, and no more evaporation occurs. Next time the wall gets cold, more condensation, but without ventilation, again evaporation is compromised. Result - dampness builds up.

Windows run with condensation in the winter, and I’m fully aware of it. We’re changing windows from metal very old double glazing to UPVC modern glazing shortly.

changing your windows might make their surface temperature warmer and reduce condensation on their surface but it won't remove water vapour from your room air. That water vapour is likely to find somewhere else to condense on to. If your room air is too humid, change your lifestyle, get a dehumidifier, or increase ventilation.