Bella, With out giving too much thought at the moment, I would think your problem is insolation, there does not seem to be any, or very little of it, its when the warm air in the room meets the cold air under the roof, hence, the condensation.
There is one way I know which may help, that's to put up a new ceiling under the old one, if you own the flat then you can do it your self with one or two friends. First, find the old joists in the old ceiling, you can do this by getting a nail and knock it into the ceiling every 2"-3" then it gets stuck in to somthing solid 9 times out of 10 it will be the joist. Do this all over the ceiling, ( along the sides by the walls) mark the ceiling pen/pencel/felt tip pen. this is so you will know where the old joists are. Screw new lumber( thats that nice word again) 2"x3" plained timbers through the old ceiling to the old joists. Start in the middle of the room ( Important). The center joist ( all joist now on will be the new ones)in the dead center run a pencel line down the 2" side This is where you will place the new plasterboard to,. I dont know what size board you use in the States so I will say 2ft wide panels By now you have fixed the center 2"x3" with the 2" side down that has the pencel line on it. Fix the next timber to the left/right with the 2ft in the center of the timber or 2ft hart to hart, keep doing this till you run out of ceiling, you should have a bit left over at each end of the room x inches. All the timbers you have fixed up to this point we will call main joists, now you will have to start again but this time the centers will be 12" these will go between the main joists,( the main joists are very important because this is where the boards will fit together at the 2ft centers) the other joists are only to stop the boards from sagging in the middle. OK, so now you have all the timbers screwed in place ready for the next part."Insolation". cut some 3" polystrine ( thats white insolation material) if you use fibreglass you will you will be scraching for a week, "dont ask how I know" now cut the poly between the joists and it must be a tight fit with no gaps or small spaces.this will stop the cold comming in to the room,. Cut your board 1" less that the size of the room, start in the middle and work your way to the end where you should have a space, you can now cut a panel to the size and screw it all into place ,screw at 8"-10" apart including the center of the panels. PS: before you start with the paneling mark the wall below each joist, this way you will know where the joists are when you start screwing the panels. Now you have the new ceiling in place, but you will have a space between the wall and the end of the panels ( I hope) the space is for any problems you may have if the walls is not straight, The Finishing touch ....screw a lath 2"x 3/8" around the walls flat against the ceiling and filling any holes with filler. Now this should stop the condensation. have chat with other people, and see what they think. John 2000.