Hi , in the past ive had success buy securing the frame in a vice (use ply wood in the jaws for frame protection) then use a large hammer and hit the top of the seat post into the frame, just to get the post moving (put a mark on it first to check for movement) . once you get a 2-5mm of movement , put the acutual seat post in the vice ( lengthways in the jaws) then wiggle the frame to see if it frees up. Ive had success like this before but you may lose the seat post as sometimes it gets really squashed, you could put a wooden dowel inside first to stop it crushing .