Thanks guys.
Matt, I use the "netpbm" unix tools (under a unix emulator on Windows), combined with a shell script that I wrote (with help from Bird as to which of the netpbm tools to use).
It creates a red/green/blue directory, takes each colour bmp, extracts the red channel and creates a bmp in the red directory. Same for green and blue.
DP uses "aviraw" to achieve the same result (a split of colour channels). I haven't used it so I can't go into detail about "how" to use it - but i'm sure DP will (he's great like that
).