Thanks for the reply, went and bought that exact extraction screw kit in the link, it was the smallest one. Unfortunately even they proved too difficult to get into the back of the broken screw and drill a hole for the extraction tool. It kept slipping off and consequently making the hole  bigger/wider around the broken screw.

I think I am going to simply have to drill a new hole right next to it for another post. As I havent finished all the other post holes for that plastic - just two, I might still be able to make it work. I have done a test fit and it looks possible.

I might also make sure I make the predrilled pilot holes a little wider to make sure I don’t snap any more. I was going to buy brand new posts but noted these ones are about $5 each and I need 15. Ouch!