I seem to remember the tailplane screwjacks were playing up and required maintenance. As there was not a spare, it would involve taking each plane out of service for the duration to do the job. Dansons Field happened at just the right time, and they could swap over the tailplane assemblies, or screwjack assembly, can't remember which, and keep the aeroplanes in service while the fleet took it in turns to be done. It wasn't called the 'Iron Duck' for nothing!
Can anyone say who was involved in the double engine failure LHR?