Whilst a table saw would be a better idea, providing you have a rip fence for your circilar saw it is doable. You need to support the Pendock do that it can't move whilst you are cutting. You need s sharp, fine tooth blade. You need to add some form of extension fence plate (e.g a piece if hardwood ply or an offcut of planed hardwood) to increase the size of the fence. The base of the saw must be clean and burr free so you don't mark the Pendock. To get the saw to run more smoothly, spread a little candle wax or wax furniture polish on the underside of the base plate and buff up. Helps it run less jerkily
If the batten is 22mm thick and the floor is 18mm thick (i.e 40mm in total) then 35mm screws should work without the need to test for joists