Thanks Paul. I can't even seem to find an online reference to a film with that title. Weird.
I did find something that might also be what I'm looking for: The 1960s BBC series "Theatre 625". In 1968 they presented Athol Fugard's "Mille Miglia", a color teleplay which showed Moss and Jenkinson's preps for the 1955 race.
Theatre 625 referred to the "high definition" 625 scanning line technology for the British TV system of the time. That HD system was taking the place of the previous 405 scanning line system, which had operated since around 1940.
Unfortunately, altho' there are lots of references to that Mille Miglia broadcast online, I can't find an online (or hard copy) of that specific broadcast itself.