There is a bike rack outside Leopold Square. But it was full when I arrived there at 3pm on Thursday 6 Sept 2012. So I locked my bike to the railings outside the Aagrah Restaurant on Leopold Square. When I came back to it (after a lovely afternoon tea in the Leopold Hotel) someone had locked another bike to it. After frantic asking around everyone I could see in the square (embarrassing!), I found that the manager of Aagrah had done it. He was most unpleasant about it, saying (I believe correctly) that it is private property and that I could not park a bike there. There was no notice anywhere requesting bike users not to do so.
I gather that the issue has been taken up with the management - so if whomever did this on my behalf would like to comment, I'd be grateful.