Try using plenty of penetrating oil and leaving overnight for starters, then you could try using a locking wrench such as mole grips or banging a smaller 8mm socket onto the head. Alternatively a company called Irwin make bolt extractors which are like a socket with reverse cut spiral blades so that as you try and undo the bolt the blades cut into the head and get a grip. It will totally ruin the head and you will need a new bolt but I guess it's already ruined if it's rounded anyway. The set I have only goes down to 10mm but they may do a smaller set I don't know. Try googling Irwin bolt extractors and see what comes up. Hope this helps and good luck mate :thumbsup:
slot the head with dremmel & use flat blade screwdriver to turn out or cut head off leaving treads sticking out when case removed...then turn threads out with mole grips
