We really should have a "Spot the Cauldron" competition with a decent overhead shot of the stadium and surrounding areas. We each put our mark on it and closest gets bragging rights.
Therefore, I opened this thread. Here's how we can do it: Download the satellite picture and add your mark where you expect the London cauldron(s) to be, together with your nickname. Then re-upload it on some image sharing website (I used tinypic. com, for adding the nickname I used the font Arial in 9 pt in Microsoft Paint). If you expect the same cauldron location, simply add your nickname behind or below the other(s) but please be careful not to hide any area other members expect as cauldron location with your nickname.
My guess is a barge on the river next to that concrete plate. It could have a telescopic tower on it and be connected to a gas pipe below that plate.
I have to admit, though, that this is a very vague guess -- because meanwhile, I'm actually totally clueless where the cauldron could go.