You don't mention the height of the room.
Assuming 8ft then aprox 27 plasterboards, 10 bags of Drywall Adhesive and 10 - 12 bags of Board Finish. About £250 - £300 for materials alone then.
You also don't say how many windows and doors there are. Are there just the four external corners or are there any internal corners too? Window and door reveals will require angle beads - a quid or two each for an 8ft bead.
How many electrical fittings and other bits that can cause a job to be fiddly are there?
For labour - difficult to say without actually seeing the job. Depends on the above but as a ballpark figure, to fit plasterboard to ceiling, dot n dab plasterboard to wall and skim the lot you'd be looking at 3 - 4 days work - I'd charge around £1,000 for that plus materials so about £1,300 all in. Like I say, difficult to say without seeing it and it may be that it's easier than I've anticipated and therefore, if it's definately only 3 days work then more like £700 - £800 for labour.
To be sure you're not getting ripped off get three quotes and bear in mind that the cheapest isn't always best. Ask your plasterer questions - how long have they been plastering, will they be on their own or have a labourer or other pasterer with them, how long they expect it to take etc. Also, if they can start immediately then find out why they've got no other work on (even with the economy the way it is there is still worked queueing up). That's a relatively big ole ceiling there and an inexperienced plasterer is likely to make a right mess of it.
If you're confident about fitting the boards yourself and are confident that you can get them nice and level and nice and plumb then you could fit them yourself and just get a plasterer in to skim. You'd then be looking at about £400 for labour and £50 - £60 for the plaster.
