Argh, I'm a fool. University Cycles lent me a bike (for nothing), I didn't take the key out of the lock that was in the basket¹, and when I got to work the key wasn't there anymore. I'm more of a fool for not attempting to fix up my bike last night (i.e. take the broken link out of the chain), and then not taking the old chain to the bike shop (where they could have done the same).
¹ Yes, basket. I felt a little self-conscious cycling around on a dutch bike with a "ladies" style frame and a big basket on the front. :-)