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The aircraft G-ZBJK got to Montréal OK from the LHR-YUL sector, some 14 minutes early in fact. But that aircraft appears to still be there. It looks like passengers going from YUL to LHR were held at the gate, not boarded, up until midnight local time, then it was cancelled at 00:03 local. This often means there was a technical issue and the local engineer couldn't fix it before the crew ran out of hours.