I remember walking back from Midnight Mass with my family on a very frosty Christmas morning in the early 1970s - no traffic at all and the air was still. We distinctly heard a jet spooling up and taking off from Gatwick a dozen miles to the south; I've always assumed a freighter 707 or similar as I seem to recall pax flights were virtually non-existent on Christmas Day back then. Never heard one since, always too much background noise.