Wet screed drying nightmare

[Angharadcc]

I have seen your responses to questions on the tiling forum and wondered if you could give me some advice?

I have recently had an extension done with a section of UFH with Gyvlon screed used at depths of up to 65mm. The problem we are facing is with drying. After more than 100 days the moisture readings my floorer is picking up is more than 4% in many patches but in others it has dropped to zero. Within 10cm distance on the floor the level can go from below 0.5% to above 4% which seems very strange. The engineered oak flooring I am laying cannot go down until the entire floor is less than 0.5% moisture content... We gave the UFH a blast for about a week with a dehumidifier in place also taking I would say half to a litre of water overnight most nights. After all that the moisture readings are completely static in the high areas. The floorer uses a Tramex CMExpertII meter but was reading moisture in % whereas when I look this up there seems a specific option for a gypsum scale so not sure he may be making mistakes...

Plastic bag tests where you tape one down to a patch and run overnight still shows a slightly darker patch underneath but show no visible moisture on the plastic at all now whereas at the beginning there would be clear evaporation of water on them if left overnight with UFH on.

Reading online there are suggestions of grinding off top layer (laitance) or using liquid DPM to get flooring laid.

I wondered if you had any ideas here particularly on the patchiness of the moisture readings and the applicability of the meter my floorer is using.

I have my doubts that the UFH was switched on slowly enough and think they may have had it blasting out quite quickly. It is clear online that this would not be a good idea but not what the consequences might be... It seems unlikely there can be a leak as it was pressure tested and the boiler is not losing pressure.

Any suggestions or advice would be greatly appreciated! Thanks in advance.