Use 12.5mm boards; nobody really uses 9.5mm anymore. If you’re having it plastered use square edge boards, taper edge are really for tape & fill but I would not advise that.
Stagger the boards across the joists & fix with screws ever 200mm - 230mm max. If your joists are spaced more than 450mm apart, you should really use noggins to support the edges or you may get cracks. The board joints must still be tapped whichever method you use.
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