Harry Bloomfield wrote

The interesting
part was that they laid a new floor on top of the old one. The new
floor comprised a layer of what I can best describe as a plastic roll
material, a little like egg boxes in shape.

The egg boxes raised the floor up away from the water and allowed the
water to freely drain into the sump. The new floor was then laid on top
of the plastic.

You're probably referring to Newton System 500 or Newlath 2000
http://www.newton-membranes.co.uk/System500.htm

Peter