The heads of the Allen key bolts that attach my rack to the bike, broke on me last night. They were fitted 3 months ago and anti-seize grease was used. They're aluiminium. Question is how can I remove the remaining headless bolts from the bike mounts? I've tried WD40 and then twisting with pliers, but no luck. Don't want to start drilling i case I make a mess of my frame?