Thanks ATCO 2.
Then WTF is it? I see it most days and it nearly always follows the same route, IE south down the Blackwall Tunnel Approach, over Blackheath, circles Lewisham and then heads off towards the Dartford Crossing.
Sometimes if traffic is grim, say in Sidcup, it will circle that area. It's always low (1500 feet or so).
Whatever it is, it follows the traffic hotspots.