take the strain off with the other bolt as stated above.
but to get the rounded head bolt out, get some valve grinding paste in there with the Allen key, that will help it bite and should remove it with less fuss. grinding paste is also known as lapping paste. works with rounded screw heads too.
having an imperial set of keys to hand can also help, as they might fit the rounded hole slightly better.
take the strain off with the other bolt as stated above.
but to get the rounded head bolt out, get some valve grinding paste in there with the Allen key, that will help it bite and should remove it with less fuss. grinding paste is also known as lapping paste. works with rounded screw heads too.
having an imperial set of keys to hand can also help, as they might fit the rounded hole slightly better.