Well last night a 23:30 hours, I saw one of the Met police's EC145s hovering pretty high (though not quite in the cloud) without a single light on, not even anti collision, the heli was simply a black blob in the cloud and I had trouble keeping track of it but for the distinctive noise. It eventually moved of to the north (towards Isle of Dogs)
How do you know who it was if it was dark and had no lights on.....???