Aidan wrote:

PS
Where I have seen problems with no/defective vapour barriers is on
chilled water pipes. The chilled water is generally run at 6 degC and,
if mineral/glass fibre insulation is used, it WILL get waterlogged
unless there is a continuous vapour barrier. The water vapour condenses
from the atmosphere. The BS on pipe insulation goes into great detail
about having a continuous vapour barrier and the pipe supports/clips
outside the insulation & vapour barrier.

This is relevant, in that 6 degC is quite warm compared with the winter
temperatures you might expect in most loft spaces and the same
mechanism is causing the condensation.