You should do the substrate one first then the wetroom one. I've done both and they do overlap on certain things but the substrate one is the most informative I think. They have a new wetroom tray 1mtr x 1mtr with a center outlet, easy to cut and work with as it comes folded in half for easy transportation, with matting already fixed to it for £70 something including vat retail price. I was blown away, I will be using there trays from now on I reckon.