This is a new thread about dilemmas that are encountered in serviceing locks. (came up in another context, but I think it will be useful) Obviouly locks in england or europe will be different in the breakdowns and other servicing problems, so maybe the thread should be divided to areas, but anyway, here is my contribution to start it off.
Third floor apt, no possiblity of NDE except by the corridor door. key spins the cylinder, but is not moving the bolt. the tail piece has failed.
You can try to disinstall the lock by drilling for the mounting screws, or perhaps working under the door lasso trick to see if the thumbturn still operates the bolt.
In the actual case in question, the bolt itself was broken, the pawl was mounted in a zinc casting that had broken and fallen to a blocking position behind the bolt inside the steel chasis of the boltworks. the actual solution was luck, a sawsall on the bolt did not cut the bolt much, but it shook the blockage loose and the bolt could be withdrawn. took about an hour and a half. Tail pieces on cylinders fail from cheapness and wear, and incomplete reassembly. they make for real dilemmas.