FORGET HEAT............... for xxxx sake...... This was an everyday problem with No5's in the far east due to pillocks before-hand not doing what they were instructed to do. Take the bull by the horns. Butt in a vice, spanner on the square shank -o or socket and unscrew it. It WILL come, believe me. When you replace it grease the xhit out of the new bolt and inside the butt and that's the end of the problem for ever more.