Right, many thanks guys. Just popped home and fixed it. Swapped the cables over so they were plugged into the right ports on the amp, and also un-pressed a little button that should have been out according to the amp instructions and I now have glorious sound coming from all 8 speakers

The headunit seems to forget it's settings every time I turn off the ignition, but looking at the comments on the site I bought it from that looks like a common fault caused by incorrect loom wiring - just need to swap the red and yellow cables over I think.