First of all, is your flat roof a "warm", or "cold" flat roof? In other words, does the insulation lie on top of your ceiling in between your roof joists("cold"), or is it positioned on top of your roof boards (the ply boards that are placed on top of your roof joists(warm))? If you have a cold roof then you can either:
- Pull down your ceiling and replace the existing insulation (but beware you don't restrict ventilation through the roof space, or you run the risk of condesation in the roof rotting your joists and coming through your ceiling boards).
- Install a suspended ceiling, either plasterboard, or acoustic tile (we'll come back to this later*).
- Convert your cold roof to a warm roof by taking up your roof felt etc and putting an appropriate insulation between the roof boards and the felt (+ any vapour barriers deemed necessary). Then relaying the felt.
*With a suspended, or lowered ceiling, you must consider more than just how to secure the 150mm 'batten', since timber 150mm isn't batten its a ceiling 'joist'. Joists are a functional and structural part of a building whenever they are under load, so, must comply with building regs. Consequently, joists must be installed according to regs, which will involve building them into the internal skin of your cavity wall, or position them using joist hangers. Either way you will have to break into the internal skin of your property, thats assuming you have an internal block inner skin, and that your house isn't timber frame, otherwise you had best call in someone who knows timber frame. Also...you have to consider the span of a joist, as this determinse its actual size, i.e. 6"x2" or 8"x2", obviously the larger the span the larger (infact taller) the timber used. Once again building regs are involved.
Of course you do have the option of running 38mm square 'batten' at 600 csc, to run at right angles to your existing joists, across your ceiling. You could fix the batten to the joists at every point they cross, using a 4" nail, angle driven, or long screw. Then fill the cavity with some insulation and have a trade board the ceiling with insulation backed plaster board.
The easiest option you have is to install a 'drop' ceiling. using acoustic tiles, all you will have to watch out for there is that the metal wire supporting the metal grid the tiles sit in is firmly fixed to your ceiling joists. Also, unless you exclusively have wall lights, you will have to drop your rose to the new ceiling height. You could insulate above the tiles, but they are pretty effective on their own.