Thanks all - think you've confirmed what the decorator said really...

I've run a dehumidifier in there for months - and it fills, I empty
it... it fills... and so on.

It rarely gets heated - and rarely used. Better half always wanted
the doors closed at night and when we are at work (fire/smoke etc) -
but that'll change. Will also leave the windows on the vent position,
locked of course.

I like the Shriver system - sounds like it will do the job (after some
research) - and don't really want the upheaval of dry lining or
insulating the walls - but I suppose this is the time to do it (just
means the decorator has to stop what he is doing...) - unless there is
an easy way of doing it... (I'm cr4p at DIY hence my pathetic
questions on here!)

Thanks again