First time I saw it about four years ago, I heard that sound and peered out of the window assuming it was an AN-12 out of Heathrow (over Purley I'd guess it would be approaching 10000' or so). Couldn't see anything except for a vapour trail - which had straight wings... Dashed for the bins...
Two Bears at Fairford one year (the year the MiGs collided). I went on Sunday, and the flyer (presumably) stayed on the gound while I was there. Blast!