Is the only reason for a flat tyre a puncture? My rear wheel was down when I got back from holiday and I thought perhaps someone had just let some air out.. but yesterday I pumped it up, seemed fine a couple of hours later so I went to the cinema, came back, all's well.. then in the morning the tyre was flat again. Balls. Could a lose cap on the nozzle-air-tyre-thing (yes, I'm a technical type, heh) be an issue? If not, I guess it's a new inner tube!