Anhydrite/Alpha Hemihydrate floor screeds - Repairs

Hi, I'm looking for some guidence from anyone who is familiar with Anhydrite/Alpha Hemihydrate floor screeds. I've removed a small 'L' shaped section of wall (750mm x 600mm)from an area that has been screeded using CEMEX SuperFlo which I'm told is a Alpha Hemihydrate. As a result I'm left with a trench 150mm wide and the depth of about 100mm. To either side the screed approx 50mm plus 50mm insulation. Under the insulation is a beam & block floor. The question is how do I make good the which will eventually be finished with natural limestone (20/22mm). I'd thought of filling with a sand/cement (3:1 mix). Would this be OK? Would I need to use a special primer? Will a cement based mix react adversely with the flow screed? The existing screed was put down 5 years ago.
Cheers, Jim

I've been told you must patch it with anhydrite screed you mix yourself and never sand/cement. You can buy it in bags from builder's merchants apparently - although I've never had to yet.

Thanks for your advice. Unfortunately CEMEX,who manufactured the screed, no longer supply by the bag and although I believe others do its a) expensive and b) not stocked by any local merchants.
On the bright side I did get some guidence from CEMEX. They advised:
"As you may be aware Supaflo is Calcium Sulfate (gypsum) based, and will react with the cement if the two materials are in contact. To prevent this use an non-metalic strip to face the edges. Something plastic would normally be the most convenient method. If you mix a 1 cement to 4 sharp sand screed to fill the gap that will be ideal.
"