i'm getting UFH laid over 70m2 as per this diagram http://bordersunderfloor.co.uk/stfd.htm

there will be 2"x1" spacers screwed to the joists at 450mm centres,
the UFH pipes are laid between these and the screed goes in to fill in
the gaps. then the final floor surface will be T&G bamboo floated on
self adhesive underlay.

the screed is only to transfer the heat ... but i've never used a weak
dry mix (8:1 dry sand / cement) like this before and i'm wondering if
it will set and be stable enough without crumbling for me to work on
it ie to finish the refurb, plaster boarding, painting, wiring etc

or perhaps it would be better to use a stronger mix? or put down a
temporary floor?