Yeah I would go for mikey-simmo’s suggestion. Obviously attack the screws on the frame rather than the ones on the door. Also if you can carefully hammer a electricians bolster behind the hinges, I think that you might be surprised just how easy hinges can be prised off.
Once opened, I suspect that you’ll find that you have a rim lock which will be easy to unscrew and replace.
As I suspect that the problem is very likely to be that the lock has become corroded and dry inside (specially if it hasn’t been opened for a long time) before you get all heavy-handed with it, it might be worth spraying some GT85 through the keyhole first. Better still squirt some oil in if you can – and dip the key in oil. If still no joy, force the key round by gripping it with anything that you can – even if the key breaks it’s still worth trying as the lock will have to be renewed anyway, and you might find that it suddenly releases.
Finally, you could also try to get a bolster between the door and the frame and prising the two apart – it’s very possible that you will be able to move it 5mm or so which might be enough to force the door open.