Phil R.. As GR said, the days you don't see the landers is probably when they're landing the other way. They land over Fulham when the wind is from the west. If the wind is from the east they land from the other way. There are complex rules about deciding when to change runways and they may still land to the west even if there is a slight easterly wind because Heathrow operates "preferential westerlies" for noise abatement purposes. If I can help further please PM me.