I got a call to my daughter's last night to look at a damp patch at the back
of her understairs cubby-hole. The very end is blocked off creating a void -
it was this plasterboard at the low end that was damp. The floor was damp
and it had risen up the wall and plasterboard. The incoming plastic mains
water pipe enters the cubby-hole through the blocked off end.

I decided that the area needed some ventilation and some trimming of the
plaster to keep it off the floor. I cut a hole in the plasterboard to fit a
vent. On taking a good look inside I pulled out some plasterboard off-cuts
(typical) and concluded that the damp had been caused by water coming up the
flexible pipe duct that the mains pipe is carried in.

The area is fairly damp and I am assuming that heavy rain had entered the
pavement stop tap shaft and had been carried to the house where the mains
water enters. A bit of wind in the right direction may have been responsible
for pushing it up a bit at causing it to overspill onto the floor in the
void.

Any suggestions on how to stop water coming in via this route? I am thinking
that an external airbrick at floor level may serve to dry the area and act
as a drain.