Ok thanks. I've also been looking at what screws to use on the chipboard.
Think I'm going to use TurboGold or TurboUltra 5x60mm screws from screwfix.
Joists are 175mmx75mm, and chipboard is 22mm. So the length needs to be 2.5x22mm = 55mm (so I can use 60mm), 5x seems wide enough, and...