Pre 911 I was jumpseating in a Lufthansa 737. Just north of London an A340 went diagonally over the top of us. I would have sworn that it was a few hundred feet. The captain showed me that it was, in fact +/-999ft away and therefore a legal separation distance.He said that even more disconcerting was when in a hold both aircraft banking, the distance seemed much shorter as the separation distance was calculated from centre of fuselage to fuselage and the wingtips were obviously closer.
I do remember flying on an AF 747 coming out of Paris Orly, first 10mins of the flight and I observed a FAF Transall coming into our port side, same height, getting closer and closer until both it and the 747 took avoiding action, he gently to the left, us gently to the right with a mile between us. No mention was made by the flight crew, most of the passengers seemed unaware. I assumed at the time that the Transall was under Mil control, coming out of the base near Paris (can't remember the name) and the flight plans hadn't been cross-checked with ATC. We should be thankful that the people who work n ATC are so good at their job, real life Tetris ;-) The London skies are crazy busy and that it all works so well is amazing (at least to me)