The post code for DVLA Stanmore (HA7 1BD) is not recognized at all by Multimap and TfL and is "out of date" according to Streetmap. Just checked Royal Mail and they say it's not in their database. So why are DVLA giving it out?! This is a real office that people can visit, not one of the Swansea drop addresses. (Which, ironically, are valid.)
Streetmap did, at least, give me a map, so I now know it is on Honeypot Lane near Canons Park station; and this let me find the correct bus from Edgware (I guessed, correctly, that the best way to go about this would be the 221 to Edgware, then on from there.)
I need to stop at John Lewis on the way back. So, Oxford Street or Brent Cross? The latter is closer, but by public transport, you never know, since I'd get to Oxford Street by tube. We'll see how my stomach is faring once I've gotten to Stanmore by bus. :-)