Here's the reply I got from Vehicle Wiring Products:
"Ford is a bit of a tricky area, they seem to manufacture their own parts and
don't release them onto the general market. I am afraid you would more than likely have to find a Ford dealer. Sorry we cannot help you on this occasion."
So my next stop is a Ford dealer, and I expect to cause much mirth and merriment on the part of the staff at the spare parts counter when I ask about them.