So yesterday marked the day when all 25 sheets of Celotex were used up (bar some odds and sods) Obviously battened and insulted the internal walls of 4 bedrooms as discussed earlier in the thread and then the leftovers (6 sheets iirc) went up in the loft to replace any wool under the boarded centre section and the wool had been compacted to fit in the 100mm void. So this should be roughly 2.5 times more insulating looking at the comparable u values and taking into account the compacted wool.
As only one half of my loft had been boarded I used the opportunity to finish the other half and go right up to the gables with the celotex, so have also given myself another chunk of loft space, probably won't shove much more up there but means getting the christmas decs down once a year will be a darn sight easier! Will be keeping an eye on condensation over the winter as some of the wool is maybe 600mm deep where I've laid whatever was left on top of each other and I've heard this can cause issues, but the majority is now 350mm-400mm except in the eaves where i've kept an air gap.