im pretty sure if you just go on top of the existing boards again with normal plasterboard 12.5 mm so you have 2 layers plus plaster or tape and fill the joint youd have to use tappered egde plasterboard that would be suffitient

1 layer of 12.5mm pboard is 30 min so 2 layers is 1 hour

regards,mike

you can do one of two things i.e.
(1) you can double board the ceiling with other sheets of plaster board

(2) you can use fire check plasterboard pink sheets

regards john

i would overboard the existing ceiling with fire check boards (they will be pink) skrimmed up then plastered, two plasterers should be able to comfortably carry this work out in one day, your boards will be around about £10 per board and going from the dimiensions you gave you would need 7, a decent plasterer will charge between 100 - 150 per day, i would say max price of this job should be 370 - 400 by the time you buy plaster, skrim and screws.
If your going to attempt the boarding yourself get some one to help because a fire board is a lot heavier than a standard 12.5mm board.
good luck neil.

Check with your local building inspector and ask him what he wants up to satisfy the regs then post as a job for quotes

You need to overboard the current ceiling with 30 minute firecheck pasterboards (pink colour) How do you know they didnt use these when they plasterboarded ceiling??????

5.5 X 3.5 you wll need around seven firecheck boards plus plastering i'd say two days so anywhere from £300 to £600 depending on job/location etc