not being picky or anything but are you sure it is 40mm. It is just that if it is a wet underfloor heating system it is likely to be a 15mm(ish) pipe diameter. Most anhydrite screeds look for 30mm cover to pipes so that is 45mm. The pipes are often held off the deck in pipe clip tracks which lift it off by another 5mm hence the 50 to 55mm suggested. If it is 40mm then fair enough but thought I'd check. There are some systems with thinner pipes so I may be barking up the worng tree but as you say 1 step ahead is no bad thing.