Get down to your local hardware shop and buy a screw remover.
It's a small tapered screw-like doohickey with a left hand thread.
Drill the appropriate size pilot hole into the top of the offending screw, then insert into hole, turning anti-clockwise!!!
When the remover reaches the bottom of the pilot hole just keep turning, that should bring the screw out.
Big Softy, June 2006