Live in top floor flat. Roof deck is made of 6" concrete with 1"
polystyrene insulation with screed and asphalt on top of that. Very
cold in winter, much condensation. Summer very hot night time. Want to
insulate on inside as we suspect the polystyrene is partially missing
when re-asphalted a few years back.

Next door neighbour, same problem, has been quoted to supply and
install a MF ceiling grid, 50-70 mm rockwool insulation, 12.5 mm
plasterboard, tape and joint ready for painting.

No mention of vapour barrier in quote !!

Questions/help needed.

1.If do get vapour barrier installed, the roof in my opinion would
then get even colder in winter and might make condensation form on the
existing ceiling which could then drip spoiling ceiling paintwork.

2. There is no thought given to ventilating the void, if were
feasable.

3. Presume water vapour, over time, would work its way in between the
new insulation and the concrete roof deck ?

4.