Well its my first time but I've spoken to three plumbers and 2 ufh companies and they weren't phased by it so must be a thing.
With respect to the screed, I sought advise from the ufh company after I realised my concrete wasn't high enough and they told me there was no right or wrong way to do it but regular screed was out of the question. they said I would need a specialist thin screed with fibre.
I rang some local screeders and they wanted an arm and a leg to come out. The mapei screed was suggested at this stage.
there was a concern raised as to whether or not the Mapei screed would be OK at just 12mm above the pipes. This is a valid point. I'll ring Mapei about it in the morning.
If someone can suggest another way to get me up 42mm to my finished floor or a disadvantage of the renovation screed, the 12 mapei screed bags are probably still returnable.
Thanks for all the help thus far.