Kevin wrote:

If I could be bothered, and thought I'd be here for a few more years, I'd
either re-roof it in insulated box-section steel or try anti-condensation
paint, which I understand works by providing an absorbent layer.

I'd not heard of anti-condensation paint, so thanks for the mention. A
friend's garage (sectional concrete with corrugated metal roof) is plagued
with this condensation problem: perhaps a specialist paint would help. I'll
investigate.

Bert