Was the original construction a complete ply sheets above the trusses which were stuck to the ply sheets and then sheets of ply stuck to the bottom of the truss which would act as a box section and vastly reduce the amount of sag and as you say the 25mm celotex would also increase the rigidity. If that is the case them increase the amount of cross pieces and that should be OK. Have you already secured the cross pieces or are they just dry fitted for assessment.
Hi Squiffy
Yes, so above the trusses it was 4mm ply then the trusses glued to the ply then rigid insulation between the trusses and then vinyl lined ply sheets glued to the underside, please see below images.
No, I haven't fitted the cross pieces permanently yet as I still have to take them back out to get the ply above them, do you think the strength will come when it's all glued together?
My main concern is that the sag remains once all glued together and then I've got a real problem on my hands as I'd have to wreck all of the insulation, the plywood and the finished ceiling to put right.