The aircraft had departed on a scheduled passenger flight from Milan to London Heathrow Airport, with an unserviceable No 3 Air Data Inertial Reference Unit (ADIRU). On final approach to Runway 09L at London Heathrow, in Instrument Meteorological Conditions (IMC), the Inertial Reference (IR) part of the No 1 ADIRU failed, depriving the commander (the pilot flying) of much of the information on his Primary Flight and Navigation Displays. ATC required the aircraft to go-around from a height of 200 ft on short final approach due to another aircraft still occupying the runway. The co-pilot, who had been handed control, performed the go-around and the aircraft was radar vectored for a second approach. The crew then turned off the No 1 ADIRU whilst attempting to diagnose the problem, contrary to prescribed procedures. As a result, additional data was lost from the commanderís electronic instrument displays, the nosewheel steering became inoperative and it became necessary to lower the landing gear by gravity extension. The aircraft landed safely.