Shining a torch inside I could see a lot of rubbish and rubble, the remains of two or three long-dead starlings, and a live pigeon. After leaving it alone for half an hour or so the bird still hadn't come out, so I got a crowbar and enlarged the hole in the grill as far as possible. After another half an hour the pigeon finally poked its head out. In hindsight I should have tried to trap it in something at this stage, but I was worried it might try to go back in so I let it fly out unhindered, and it immediately went and sat on top of the highest shelf it could find.
The next two or three hours were spent trying to persuade the thing to fly out of the window. I chased it around waving a stick. I shut all but one of the blinds and switched the lights off so the only light was coming from the open window. I left it alone for half an hour (it sat and preened itself then went to sleep). I chased it around again. I tried to catch it by throwing a sheet over it. I chased it back and forth, continually trying to direct it towards the window (it kept seeming to consider flying out, then changing its mind and landing on a ledge above the window). At one point it tried to fly out and bounced off the part that doesn't open. It did eventually fly out but only after I'd wasted the best part of an afternoon on it.