There wasn't much I could do apart from setting out a bowl of water and keeping the door of that room closed and hoping that she'd turn up - which she did tonight, in the flat below mine - in fact two floors below mine. Had a worried phone call about ten o'clock, they could see her hiding under their TV stand. I have no idea how she got down there, but apparently one of the cats had been acting strangely all day, so she'd been there a while. It's actually the second time an escapee has made it down there, so presumably there is some sort of direct route - probably where one of the central heating pipes goes through the floor or something. I shall have to investigate.
Anyway, it all seems to have ended well - I got her out fairly easily, and she appears to be unharmed by her adventure. I'll let her settle down tonight and feed her tomorrow.
And buy a potted plant or something for downstairs...