[spinlondon]

Sounds like you have over heated the srews causing them to become hardened.
You need to get some Cobalt drills preferably left handed ones.
Snap On do a set of 5 varying sizes, and can be obtained via a Snap On dealer van or via their website.
I say left handed because sometimes when drilling out snapped bolts etc. the drill will catch the fastner and draw it out.