Sorry for the massive delay replying - real life got in the way a bit.
Thanks, yes, you're right, I meant the Cut Silence option - I'd forgotten exactly what it was called but knew it was related to gapless playback.
If I understand this correctly gapless playback depends on my files being encoded in a particular way. Is there a formal way of checking whether a given file is encoded this way? Or, say, if files play gaplessly on Windows Media Player, does that mean they must be encoded for gapless playback? Or does Media Player have a way of faking gapless?