Morning
@Andy Rhodes !
I'm a little tight on height yes! The new subfloor is flush with the adjacent screed and the wood floor. Both substrates need a decoupling layer so Ditra would be the best bet from herein.
I built a couple more sleepers over the weekend, went back around and packed out with slate and plastic shims for small adjustments. Blocked in the frame at opposite corners. The max clear span now is approx 800m at any point. Ply joints have a small expansion gap.
Probably give it a more rigorous test this evening as I left the mortar to go off but on first showing it was much improved the missus did a bit of jumping mid span on a few joists and the laser was moving at most 1mm and nothing at all if you're walking around normally.
The length of the tiles are 1200mm, so when you run them perpendicular to the joists you're actually spreading the load along 4 lengths so I'm not worried about any interstitial deflection between them due to the ply thickness.