Long story short - joiner laid the plywood sub floor above the OSB in readiness for the resin pour next week. One layer of 9mm glued & screwed with 12mm laid on top. We then discovered a squeak which we investigated today by lifting the ply in the  affected area. Looks like the OSB is moving slightly against the joist, it did get wet when exposed over winter. Anyway, that's not the pressing issue! Some of the UFH pipes come above the OSB (to allow them to return over the pozi joists) an