I was on board. Fairly uneventful take off and climb-out, although captain stopped climb at about 8000ft. Did a few lazy racetracks over the North Sea to dump fuel and then returned to LHR at about 2315Z. Announcements kept us informed of progress and reasons for return. All very good. Packed off for a night in airport hotels. According to the letter I recieved from the airline this morning it was "...due to a problem with a faulty switch on the cargo door". The letter goes on to say that the "...engineering team were recitfying the problem, however, the flight deck crew unfortunately would have gone over their duty limitation hours if the flight had departed last night". The new flight number is NZ6838 (showing on the boards here in T1 as NZ038D) due to leave at 1400, arriving Hong Kong 0930 (5th Nov) before departing for Auckland at 1115 arriving 0300 (06th Nov). That said, the screens have just gone from saying "Gate opens 12:30" to the dreaded "Please Wait". :-(