If you werent so far away id say id plaster it for you for £100, only 1/2 days work. and probably tack and plaster for £250 if you supplied the gear. (cash of course)

You want to use 1/2" board, make sure to over lap the joints, start in one corner and work out.

Use 42mm dry wall screws (they will be black) just watch when you put them in that they dont go too deep, want to be about 1mm from the surface.

Some dry wall boxes come with a specil screw bit, i think travis perkins normally does

To cut the board, lean it on a wall. get your tape measure to the desired length. pinch the tape at the tape measure base with one hand and run this along the wdge of the board, whilst in the other hand you have the end of the tape and the sanley knife you hold on to the metal end of the tape. this is so much quicker tham drawing lines, its what we do "in the trade" anyway. once you have scored the line hit the joint and it will snap then run the blade down the other side!

YOu may want to see if u can get another quote for someone to do the lot on a cash deal, because no offence but there is nothing worse than trying to finish off a joba nd make someelses work look good!

If u do want to do it yourself, you could use the tapered board and "joint" the celing instead of plaster.

this may be the best way if u want to do it all your self!!

tack as before with but with the 1/2" the tapered board, run your scrim (plasterers tape) over all the joints.

then mix up the jointing compound (you want to use the "easy fill" made buy gyproc in a light green and white bag) i recoke on bag will do you, its about £18-20.

simply apply witha trowel to the joints, leaving it slightly proud of the board, the next day when its dry use a palm sander to sand it flush.

then simply paint the celing!

Remember it is messy tho due to dust, but may be the better option if u want to do it all your self

nb; Only mix 1/4 of easy fill at a time as it goes of fast!!

also if ur tacking buy your self use some DEAD MEN STICKS, basically 2 bits of skirting or batton, about 1" longe than the height of the celing.
offer the board up to position, then use the dead men to wedge the board at each end buy kicking the bottoms into the floor. again its what we do in the trade. but id recomend u get a mate, may be safer