West London VW at Power Road (Inchcape).  I took the car for a service and it was one of the items identified as needing attention. I think someone there must have made a mistake.  I'll give their Parts dept a call.
I just checked what was on the service report (ie what the technician completed) and, as it turns out, it was actually the passenger side one that was missing.  So, I think that's part no. 5K0 805 413.
Thanks for the info.  Much appreciated.
cjp64