"GT" gurgled happily, sounding much like they were saying:

Double red lines? What are they?

They signify "No Stopping" on London's designated "Red Routes".

http://www.tfl.gov.uk/roadusers/fine...ons/10185.aspx

I have read that page, including reference to the PCNs, but are they
actually a recognised, enforcable highway code entity

Did you bother to look at the HC before asking? If you had, you'd find
them in the "Signs & Markings" section.