Is it one of those that looks like a shruken speed camera on a really high post?
If those are speed cameras...there's another - A1 Archway Road, facing south just after Highgate Gyratory (i.e. north of Highgate Tube). no idea how long it's been there...only spotted it a couple of days ago, and it's a 30 zone coming straight after a 40. Oops
Jo