Hi John,
I recall that Lee's original build manual for PAW stipulated that the USB to RS232 cable needs to have the PL2303 chipset installed. Could this be your problem?
Alongside this statement is a picture of a cable assembly having USB at one end and wire pigtails at the other end, rather like the Farnell product that Wobblewing posted a link for above. Not sure that the Farnell one is compliant with the chipset requirement, also Lee suggests this type of cable assembly can be obtained via eBay or Aliexpress for less than a fiver.
I haven't managed to find such a cable on eBay, they are more commonly USB/DB9.