Just had a reply (once I paid the excess postage!) from the guy who suggests that the little Marconi-Stanley engine may have been part on an APU on the Armstrong-Whitworth Ensign aircraft.
Evidence is somewhat circumstantial but here goes:-
The chap he had the engine from some 30 years ago worked on the construction of the Ensign in the 1930's and also on the scrapping of eight aircraft returned to Hamble in 1947. It is at this time he acquired the engine.
He also sent a copy of an article in 'Air enthusiast' Feb '72, part of which reads reads:-
"An unusual feature of the Ensign's equipment was that is included a rudimentary APU in the form of a small petrol motor, located on the flight deck and permanently to a generator so that a supply of electricity could be maintained for the radio and cabin lighting when the aircraft was on the ground"
I know there our several vintage aircraft enthusiasts in our number, what say you?
A few pics of engine in Internal Fire gallery:-