Still no firm ID on engine, but somewhat against expectations, I have had a reply from Westminster Council. Their archivist was able to tell me not only that Upper Marylebone Street became New Cavendish Street in 1937, but also that Kelly's directory listed a Mr. Robert Stewart Inglis, Motor Engineer, at the address shown on the engine in 1928 and 1930. Thank you Miss Alison Kenney.
Now all I need to do is work this up into a fascinating piece for SEM!