Thanks both. Blackjack is a tar-like bitumen that I assume acts as a
waterproof barrier between the two layers of concrete. To be fair, the
two ends of the run are dry, and where it wasn't covered with carpet
it is now dry, so I assumed that his method worked OK, and it was just
concrete under the carpet that was not dry as it was covered.

The original plans had drains going around the house, but this would
have meant the gradient was to shallow as the run was too long. In
order to do what we wanted, they had to go across the lounge (and this
was with the approval of the Building Inspector).