Dizzee.... Let's put it this way... if there was any aerial activity around Biggin Hill in the middle of the night ATCOs at TC would become curious. I sat in front of the radar for many, many years and you get used to what shouldn't be happening. OK, low cover of the NATS radars doesn't go down to 50ft at Biggin, but you'd sure see it before it descended that low. The police helicopters talk to Heathrow Director all night so ATC knows where they are..
It's beyond all belief that there would be a number of such incidents without ATC knowledge. Believe me, if anything untoward happens the phones start ringing....