I'm as puzzled as the rest of you. I saw it circling anticlockwise centred on about Tooting (S London) I would guess, between around 20:15 and 21:25 BST, apparently climbing steadily as it circled (although the circling/turns were slightly irregular) and I think it was display no nav lights at all. It disappeared from view before it got too dark to see, heading approx SE.
Would this have interfered with Heathrow takeoffs (they have been on Easterlies this evening)?