Cheers @TooTallTim and @Bobbo . I'm confused about why boards need to be perpendicular to joists though? I'll end up with far more unsupported edges that way vs if I run them parallel. Are timber battens half and half overlapped at the supported edges good enough or do they need to be attached to the joists too.
I'll have a second person helping me. Was planning on just using a drill driver and one of those drywall bit holders to get the screw depth correct. I'll be doing the job bit by bit over a couple of weeks or so don't really fancy hiring any equipment. I was going to make a couple of T shaped upside broom things to help wedge the boards in places while I drive the screws in.