Hi Millsy,

I have removed broken bolts, screws by centre punching the head, drilling out the centre using a drill bit slightly smaller than the screw. If you use cutting oil or wd40 on the drill bit it will drill much easier. Not sure what this might do to the playfield to you would need to keep this covered up. Once you have the screw hollow you can use a screwdriver or punch to collapse the screw inwards and it should come out with a pair of pliers. You may also need to sharpen the drill bits it you find them blunting after a couple of goes. 

regards Derek