Thanks to my brilliant mate for helping there - he's done screeding before. 75mm of 3.2:1 sharpsand:cement (because the sand came in 20kg bags, cement 25kg, which worked out at that after deciding to use 4 bags sand to 1 cement[1] plus 2 big handfulls of 20mm polypropylene screed fibres as it's sitting on celotex.
[1] and a few checks of various sources showed the density of sand and cement were very similar.That's been down a week and feels fairly well set in general. It was drying out a bit fast in the heat, so after gently hoovering off the loose sand fibres and trimming the edges of any bumps and excess DPM, have use hose on superfine mist setting to fully dampen it right through.
That was something I did in the conservatory when that was screeded and I believe it helped as that ended up like iron after the final bits of loose sand were brushed off.
Screeding is still a bit of a dark art to me...
It's now solid enough to walk on but occasional patches of the surface are a little friable. Having read screed takes a month to fully harden, intend to leave it alone now before working in there any further.
The method of holding the drain in place worked very well:
In the hopefully unlikely event that any bits remain friable (seems to be odd parts where the larger pebbles in the sand gathered) I'll give the a touch of SBR to stabilise, but I'm hoping it will harden up of its own accord given the full time.