I normally find the joists and mark them on the wall they are normally 401mm or 601mm apart then just start lifting boards up, try and stagger the joints and I also put a tack between the boards so that the distance is kept consistant.
I have 2 x Easy props that help supporting the boards while I tack them.
The main thing to do is make sure that before you fix the board that you can get a good fixing on the next board. Find the centres by making sure that you can get 3 good fixings and use the middle 1 for the line.
thanks lads ... i wish i could rip it down but the bloke doesnt want the mess ..what diameter board is best , taking into account the added weight , is it 9.5mm... also can you give me some advice on power tools for fixing the screws , which are the best ones and prices ... many thanks