<<I think a Lufthansa 720B got on the glidepath for Northolt a couple of years later but realised the error and went around (to LHR )>>
Northolt did not have an ILS in those days so approaches were largely visual. The closing heading for the then 23 ILS at Heathrow was around 250-260 degrees and if the aircraft was in the right position, as it turned on to the heading the Northolt runway came into sight dead ahead. Whereupon, disregarding the Heathrow ILS and various other factors, some crews just "went for it". The fact that they were a couple of thousand feet high didn't seem to worry them and it was difficult to convince some that they were not looking at Heathrow..