just for your information, the air conditioning pack was to blame. I work at BAMC and although not on shift, I have spoken to the guys who handled it that night and the pax were bussed to the terminal where they were taken to a hotel. Fault finding was carried out and a turbine/compressor blade within the pack was found to have sheared resulting in severe friction/heat causing the cabin smoke. We had a 777 in for maintenance anyway and a pack was 'robbed' from it and fitted/tested ok. The pax were brought back to Cardiff and flown back to LHR where a crew swap/refuelling took place and the aircraft left for Toronto the same evening. All in a night's work for the guys involved, big well done to them too!