when i was a bit younger i used to go round picking up dumped bikes, they were clearly dumped, possibly stolen (not by me!) and dumped but they werent ever locked up. they would be in rivers and such and mostly missing a wheel etc. i dont think i would cut a bike free although i have seen a few that need it. its a shame to see them just rot away. maybe contact the police and inform them the bike is there, maybe the law would then still work that if no one has claimed it within 3 months then maybe you can free it? or maybe they will then tell the council to 'clean it up' :? many options!!!