The manufacturer is clear:
"We say up to 10 mm aggregate is OK around the underfloor heating pipes. I am also concerned about the size of 20 mm aggregate and specially when the total thickness over the insulation is only 65 mm. So it is wise not to continue with this mix."
A whole large room (living room) has already been done, and ripping it out would be pretty difficult, so it looks like I am going to live with it for this one room, but certainly will be requiring a different mix for other rooms. Fortunately for this one room, we took a precautionary measure and put in pipes for two wall radiators in case they were ever needed in future.
The problem is, my builder says this is fine, he's done it with UFH before, it doesn't cause a problem, it works (convects okay, probably not as efficiently ...), and the two guys laying the screed seem to understand the issues (without even asking, they showed me how the screed is sufficiently meeting the pipes, and he referred to the double wall construction of the pipes). It's one of those miscommunications that you cover with the builder all the details about what's to be done, but you don't actually ask "so exactly what are you putting in the screed".
To finish the surface, they say they will use a slurry screed and smooth it. Timber boards are going over it eventually.
But I am pretty nervous, as it is clear: the effects won't be obvious until I warm it up a couple of months from now. But I have to live with it for 10 years, they're off to another job in a few days.
Do I get it ripped out?