I gave a quickie response to indicate to the reporter that such sightings and his concern were fairly commonplace. I was in the OCK hold a few weeks back and there was an A310 1000 ft above and just outside us. Each time we turned our left wing went up and his right wing went down... and it looked every bit as if our wing tips would touch. I was on a smaller a/c (A320) so from the ground it could have looked horrific. Scientific explanations are hardly necessary - someone has had a fright and simply needs re-assurance that all was well.
One of the phone calls I received at Heathrow was from a guy who referred to himself as a "Senior BA Captain" who had been driving across one of the Thames bridges in central London and claimed to have witnessed a "dangerous airmiss between a light a/c and a Heathrow inbound". Light aircraft and helicopters fly back and forth under Heathrow inbounds all day long under ATC control and at that sort of range there was probably 2000+ ft between them. I advised the Captain that there had been no problems and explained that vertical separation existed between such flights. He would not listen to reason and was absolutely adamant that he would be filing a report!! I never heard any more...... wonder if he's reading this now?
Remember what Father Ted said to Dougall - "This is small but that is far away..."