It is damp on both sides of the cavity?

Are you an idiot?

To start with, if it is coming in in the rainy weather it will dry out
considerably when it's dry but when wet it will be wet as hell.

What could be constricting the cavity long enough to stop the rain
dropping past it on the inside or staying long enough to cause quite a
large damp patch?

If it is a small damp patch that is present in dry or wet spells it is
coming from a leaking pipe or connection.
http://groups.google.com/group/uk.d-...2dcabbab2c38df


There are no leaking pipes anywhere near the wall.
The bathroom is attached to the kitchen on the ground floor, so it can't be
leaking, as you suggested.

I think it was the broken down pipe, as it is directly above the wall with
the damp patch.
It was a fixed a few weeks ago, so hopefully the patch will now dry out.