span a steel beam, say 8x5, either side above the ceiling/roof joists
below
bolt 6 x 2 joist inside the full length of the beams (you'll have to
drill the beam or buy one ready drilled) and attach joist hangers to it
then suspend the new floor joist from the hangers
the beauty of this method is you can set the floor height to anything
you like (within reason) by placing the joists between the ceiling/roof
joists below

I simply don't see how this would work for me. Where do you hide the 8"
(200mm) beam? I only have 100mm to play with and need a continuous floor
with the conventional joist length being a centre supported 8m.

wouldn't a T&G floor would come apart if it's not secured ?

I presume you use PVA, like old style non-click laminate. I don't know if
you're allowed a floating floor of this size. The actual floor size would be
more like 6.5m x 4m, as there would be a dwarf wall at the front non-dormer
end. Thick skirting to hide the expansion gap and some blobs of low modulus
silicone to stop the entire floor migrating to one end. Is this allowed?

Christian.