Having actually done some digging, it appears the Mail is partly correct, though three years behind the Telegraph who picked up on this story in 2011.
Met Police spends millions of pounds on secret aircraft - Telegraph
Having heard a few stories about infringements of London's Control Zone (the first being from a friend who did it while doing an ATC Flying Scholarship out of Stapleford in the early 1980s) I would have expected some spotter to pick up on it. Turns out they did.