Forget got this from Airport technology site:
Starting in January 2005, prefabricated 12m sections were then inserted under the cab forming a steel mast and gradually building to the full height of 87m. The tower is supported by a 15ft (4.6m) diameter steel mast and three pairs of cable stays anchored to the ground.
The mast, which is encircled by a three-storey technical and administrative building, provides access to the visual control room via one internal and one external lift, plus an enclosed stairway. The tubular plated main core was shaped to accommodate lifts, stairs, services and electronics, and the three stays ensure the rigidity criteria for the radar systems.
Arhcitect was Richard Rogers Partnership and engineers were Arup.
Millennium Bridge was designed by Foster and Partners and engineered by Arup.