Another vote for the Irwin sockets from me. I've got the basic set and added the extension set so the box is now full. I do a lot of work on old British cars and they are invaluable when I'm following someone who only had a Metric socket set (or only used the sockets from one side of their set).
+1 for the Irwins - they will do the job. If you want to spend no money, then a hacksaw blade with a bit of tape on, but then you are still going to have to extract what is left of the bolt.
You could try hammering a smaller socket on to the head, but then you run the risk of distorting thingsand damaging threads or even worse, mullering the head of the bolt even more making it harder to remove.
Alternativly, try taking the door bracket off and see if it gives you more room to play.
On a similar note, I once tried a stud extractor type socket thing from MM - I think it has BB's and cams inside to grip. It exploded.
If you do need to saw it off and don't need to buy an air saw, try someones suggestion of taping a hacksaw blade up.
I have several of THESE they are OK but a taped up blade is often better.
can you not just weld a bolt to the original bolt?
you could do it so that the head extends beyond the enclosure thingy and use a
open jaw spanner.
bill