Thanks all – I don’t mind ripping the ceiling down as the room is bare (no flooring, units etc) so the mess is fine, and would save me lowering the ceiling/creating a new framework of 3×2.
There’s a 1′ void to the roof (which is adjacent to and “breathes into” a pitch roof/attic space) so I’d thought I could fire up proper loft insulation into that then just fire plasterboard on existing battens – gotta be better insulation than 30mm of polystyrene etc (which can’t be a great fire risk for insurance purposes?).
Thing is I should really do the same to the other 2 rooms under the flat roof – I’ve just had the ceiling papered in the larger of them…