i have used devi mat quite a few times and it has always been easy to instal and reliable
Simple to wire too. RCD controlled spur supplies a controller outside the bathroom, which in turn is connected to the heating mat and the floor sensor. I usually get the digital programmer (550) which is a bit of a pain to programme initially, but once its in thats it done)!
Make sure you get the 100 w matting for wooden floors, or the 150 for concrete ones.
I am sure that devimat do a video or instruction guide for fitting on their web site. One other tip is to be careful measuring the size you need - just think of the main areas you need to cover !