kevmusic,
Perhaps someone who knows about these matters will correcct me, but I think Aces High still has a DC3 (strictly speaking, a C47 Dakota) on a private C of A based at Dunsfold for film work, so that's one possibility. Is it registered G-DAKS?
If you think your musical ear detected a pair of Twin Wasps, you were probably right. Unlikely to have been cruising at "umpteen" thousand feet, though. Being unpressurised, the Dak tends to cruise at FL100 or lower. (FL070 or FL080 would be typical on a journey of an hour or more, but even 2000ft -3000ft on a short trip, like skirting controlled airspace around London.)
Treadigraph's DC6, of course, is a very different proposition...