Regarding the Heathrow/Northolt mixups, I noticed the "LHR" and the arrow on the Southall gasholder only a couple of days ago, it's right alongside the Paddington railway. It appeared freshly painted.
But that's not on track for 27L/27R at all. It is on the 23 approach (is that where the Pan Am was heading) but way to the south of an approach to Northolt.