I thought that at first. Then, after a year of doing it and never encountering these unmarked police cars, I'm beginning to think they don't exist!
Essex police seem to have quite a relaxed towards speeding. Not a lot of fixed or mobile cameras, you can be making some significant progress right past them and they'll not bother, I've driven head on towards both ANPR Evo chase cars while overtaking a lorry on a slightly dodgy bend down a hill and they didn't turn round, sat next to a blues-and-twos Focus estate at 120 with no comeback, pulled over by the police with them saying "sorry, you were driving it like you stole it" (exact words there) and they were just checking I was actually the owner etc. They just don't seem too bothered

I thought it was about 1mm by hand, but after checking it, it's dead borderline.