Mysterious black radio ops in London

I had just popped out of our Southwark offices for a moment at lunchtime when a most peculiar vehicle swanned past. It was a shiny black Ford Mondeo estate, but the roof was studded with more antennae than spikes on a Hoxton haircut.

I didn't have a chance to count them, nor take a picture, but they were arranged in a rectilinear array in what looked like a 4x3 or 5x4 configuration - so there were probably between twelve and twenty of them. They were very short, black and stubby, which means they were operating in very high UHF or low microwave bands - I'd say somewhere between 3 and 6 GHz.

This wouldn't be a direction-finding array: you only need four antennae for that, arranged in a square. It looked for all the world like some sort of very large MIMO system - but much larger than any I've seen proposed for mobile data.

Does anyone know what, who, when and why? Have there been any other sightings? Informed guesses or random ideas more than welcome: meanwhile, I'll do some research among my more radioactive pals and see if clues are forthcoming.