Fifteen minutes after takeoff from Frankfurt Airport, while climbing through FL100 the translating cowl of the no. 1 engine thrust reverser became detached, struck the leading edge of the left horizontal stabiliser and fell to the ground. The commander subsequently shut down the engine and diverted to London Heathrow Airport where an uneventful overweight landing was made. The aircraft had had two previous malfunctions of the No 1 engine thrust reverser system. CAUSES: (1) The ground engineer took actions which were not supported by his knowledge of the aircraft and, unrealised by him, were not mandated by the Master Minimum Equipment List.; (2) The ground engineer’s supervisors failed to identify that an incorrect procedure had been followed although he had communicated his actions clearly to them.; (3) Damage became evident which was probably an indication of progressive cowl failure but it was not recognised as such.”