Thanks for all the replies, chaps, most informative. Magnificent cutaways. Evidently the "Alvis Leonides Viscount" was an urban myth, or perhaps just a faulty recollection on my uncle's part. I, also, wondered about the sufficiency of four such engines - might have needed six, like MRIA! (Kidding).
In parenthesis, I always thought the Ambassador a most elegant and attractive aircraft, if a little chubby-looking, and I've sometimes wondered how differently its service history might have turned out if it, too, had been built from the outset with a pressurised cabin and four Darts (I'm assuming the Napier Eland conversion was not a success - I've never heard that there was any production run).