Just in case anyone else comes across this I got it working. It wasn't easy and took some trial and error, there isn't very much documentation out there about the inner workings of the asound.conf file but I must have read it all.
The asound.conf file needs to look like this. Obviously if your usb sound card is a different number or you didn't disable the default sound output of the pi yours might be a little differnt bit this should mean you get no errors about mixers and both game sound and background music on ES play ok all through the left channel only.
pcm.custom {
type plug
slave
{
pcm "dmix:1,0"
}
ttable {
# Copy both input channels to output channel 0 (Left).
0.0 1
1.0 1
# Send nothing to output channel 1 (Right).
0.1 0
1.1 0
}
}
ctl.custom {
type hw
card S2
}
pcm.!default pcm.custom
ctl.!default ctl.custom
