I suspect it is the XPS itself.
I have bought a second hand Pod XT live and the guitar works fine through that without the battery pack in it using the ethernet type cable.
Since I can now use the Pod, I guess the XPS is less needed - I can use batteries if I don't want to be bothered setting everything up. I will keep my eyes open for a used XPS and if one comes up for cheap will probably get it.
Many thanks for the help and advice everyone!
Now all I have to do is figure out what everything does on the pod - it might take some time....
Cheers - Jon