Further to my last post, I have removed the xen-pciback module from the Dom0 kernel, and reloaded it as modprobe xen-pciback passthrough=1
I now have the PCI device on the DomU matching the Dom0 Device usb usb1: SerialNumber: 0000:00:1a.0 instead of 0000:00:00.0
However I now have this error
ehci_hcd 0000:00:1a.0: Unlink after no-IRQ? Controller is probably using the wrong IRQ.
does this give a clue as to what is going on?
Many thanks Francis