Thanks Dave for your responses, that's really helpful.
Unfortunately there were a number of other problems with the installation, done by the builder, so I think we'll be ripping everything out and starting again with another installer.
If I understand correctly the insulation boards are actually the bottom layer that we can see and the electric matting is sitting on top of that.
I've found another couple of photos that show the insulation board, but unfortunately we don't know how everything was fixed together.
Hi.
We've had a bathroom installed with electric mat underfloor heating in a new extension. The heated mat was not turned on for a month after the tiles were laid and then the heat was gradually increased by 1 degree a day. The mats are on a timer to reach 25 degrees over a 3 hour period each...