Help - ruined slate flag floor - Advice needed on badly laid floor

[Albert]

I have a collection of massive old slate flagstones - some up to four feet long. These have recently been laid on top of a limestone screed containing UFH (not turned on yet). The guy used a 5:1 mix of NHL3.5 and sharp sand and laid them on a full bed of mortar. It looked stunning for a day and then I noticed a number of the flags rocking and sounding hollow if you stamped on a corner as if they were parting company with the screed underneath. Now another is doing the same thing. How has he got it wrong? Apart from the obvious annoyance of a solid floor not being solid, my chief concerns are that the UFH won't work properly and that the more he attempts to correct them, the more damage he does to the slates as he crowbars them up. I don't mind a 'distressed' finish but these flags now look suicidal.