My key broke off in my U-lock today. It's a real old (and unfortunately sturdy looking) model that I got from my friend, no brand. The end of the key is stuck in there really good (there IS a spare key) and sticking the unbroken end of the key in to turn it only strips things badly and gets the other in stuck in further.
I'm not above destroying the lock to get at my bike, but it's in a very public place and I have no tools. Ideas?