So that was about 75 metres away, and hedgehopping altitude. Big bugger.
The sky here can get quite crowded, what with helicopters often passing by at low level, planes using the mid distance and mid levels to practice aerobatics for airshows, and then higher up, the landing approaches for Stansted and/or Luton (depending on conditions - we're about half way between them).
(Our location - we're the square building in the centre, Tesco is the adjacent lot with the huge carpark.)
ETA Boeing CH-47 Chinook. A large helicopter. As in, 30 metres long, twin rotor, can carry over 12 tonnes load. There may be a larger helicopter out there, but I'm not aware of it. Knowing it to be 30 metres in length indicates it was probably at about that altitude.