Earthmover,
it was NO on the gas holder at South Harrow and LH on the identical one at Southall (which lined you up with 23 at Heathrow). I had a very good view of a Swissair DC8 nearly doing a repeat of the PanAm episode from my parents' house a few hundred yards from said gas holder. I understand the trick in those days was to line up the spire of St Mary's church on the top of Harrow Hill with the appropriate gas holder - no doubt someone will correct me!