Its a Beava Wondermat.
Kinda the same as the other brands (ditra, dural etc) but Beava
I've used it a fair few times, STS in Leeds used to sell it, but there's been a few reported problems in the past, so now its been replaced by Sandimat (I think).
They've tried to sell me the Sandimat instead of Ditra, but they can't convince me