Except the ones sold by Evans for their Hoy Fiorenzuola. They appear to be M4. (I was in Evans the other day looking for the very same thing and thought that they'd do the trick. Then I saw some fitted to a display bike and realised they'd be too big)Long M3 Machine Screws.
Ideally they must have springs fitted to prevent them rattling loose. However, a bit of light threadlock will do the same trick. If your frame is steel, they're definitely M3, there's no space in most drop-outs for thicker screws. Thanks for the heads-up,Tim Hall. I've never seen M4s used in there.Thanks gents. I have no idea what the frame is so am not sure offhand what size I need but have a fair selection of screws I can try until I find one that fits, then get the real thing.