At about midnight, I saw a small plane (a [Diamond Twin Star](https://en.wikipedia.org/wiki/Diamond_DA42)) take off from Heathrow, on flightradar24. It then proceeded to fly in irregular loops around North London, at low altitude, 1700ft. The plane had registration G-COBS, and seems to be owned by "[Cobham Flight Services](http://www.airport-technology.com/contractors/traffic/cobham-aviation-services/)", which provides "flight inspection services". Their web page advertises how good they are, without explaining what they do.

It seemed unusual, as such small planes don't normally use Heathrow, and flying low over a city at night also seems unusual. What was the purpose of a flight like this?