Ah, thank you ... I was just about to start a hunt for an aerial/satellite shot. I've just come back from Heathrow, and the plume/inversion effect is pretty remarkable, even on the south side of London. In the time it took me to drive from/back to Junction 5 it has spread remarkably, and it's now clearly visible as far down the M20 as Aylesford/Maidstone, though the skies are still clear in Folkestone at present. The light is very spooky, like an eclipse.