Finally figured it out - after 2 weeks of trying. Found a guide that updates the Windows XP files before install to move the USB drivers early in the setup, you have to extract and customise the USB.IN_ USBPORT.IN_ USBSTOR.IN_ and recompress them with the updated config so that windows loads the USB drivers and you can complete the installation.
Took a while to install onto the SSD but once it got there its running fine (hurrah!!)
The aim is now to run 2x 1TB drives as a software RAID (or hardware raid on the PCI slot) to have an "always on" resilient storage center at home. Windows XP 64bit too so should be well up for the task
Edit: http://www.ngine.de/article/id/8 (link to the guide for USB boot)