Tightening before loosening works, as does giving a stuck bolt/screw a sharp tap, failing that heating it, usually done with an oxy torch or similar also known as a "gas axe" although for bolt removal you obviously don't want to use the cutting tip, nowadays there are induction heaters for bolts, nuts, studs, etc.
https://www.bing.com/th?id=OVP.a4m7y...bw=3&bc=ffffff YouTube
Lukeis, I'm glad to hear you managed to remove it, you might want to put some anti-seize on it before reassembly, for easier removal in future.

