Here's the spec sheet from the screed supplier.
I'll search through my emails and find the discussion I had with Norcros.
I also got the opinion of a third party, who didn't agree with norcros that more laitence would form after the initial drying. And stated that the adhesive manufacturers need to get their s#*t together with these screeds as they all seem to have a different opinion.
FWIW, I removed all laitance, primed 3 coats with an acrylic primer, the last of which was near enough neat. Fixed ditra mat with rapid cementitious adhesive. Then fixed the limestone with rapid cementitious adhesive.
The first phase was completed before Christmas, I was there briefly last week to check up on phase 2, and it all seems OK.
The UFH has been running now too.