Ah ha, it appears that Alesis do a £100 USB FX model which gives a stereo mix, and a USB 2.0 FX model which is just over twice as much and this provides 8 separate outputs via USB which is what I need. Gets good reviews, but twice as much ;-( I read an old post somewhere that suggested that most people don't need the mixing element, just the recording element and can make do with outputting as 8 tracks to a memory card, so am now looking into this which sounds like an even better solution!