Having obtained ATC clearance, the captain taxied out from the apron, behind a parked Vickers Vanguard, and turned right on to the taxiway towards the take-off point for runway 24. A sweeper vehicle on the taxiway was some 300 yards from the Comet and heading towards it. The driver saw the Comet approaching and turned left off the taxiway; as he did so, he saw a “No Entry” Sign painted on the tarmac and momentarily stopped. He then continued slowly to drive away from the taxiway, to give the aircraft sufficient clearance but, at this point, the wing tip of the Comet struck the vehicle. The tip was damaged and had to be changed. The crew of the aircraft saw nothing of the vehicle and had not been warned that it was there. The vehicle was painted a dull grey, and had no R/T or beacon light.