aviate1138. It was actually in 1972 and I was on the way to work along the M4 with some colleagues when it went past us heading west, almost level with the car!! I believe it eventually got to 1500 ft around Compton and then turned south to dump fuel over the sea.
Later, when it returned, my buddy who had been driving and I were working the approach sector. My buddy asked the pilot if all was well; it was. He then said something like "We drove past you on the way in and it frightened us to death". The 747 crew thought that was great fun but the "powers that be" didn't think so and my buddy had to stand on the boss's little bit of Axminster the next day!!