For the benefit (etc etc), this the LHR - MIA flight from Heathrow terminal 3, which was due to be operated by G-CIVU, a 747 jumbo jet. Since this aircraft arrived back on time from Miami this morning, and is therefore on the premises, I would suspect that there was a technical issue either with Victor Uniform or with the original aircraft.It was due to depart at 09:35 (and I think they need slightly more than 3 hours to turn a 747 around at LHR) and is currently down with a 12:30 hrs departure. If you are at the airport I'd imagine they would have more information.