I have heard horror stories about Insurance companies working with Police and if the 'Black Box' omits a signal that you are speeding then a fine and points will be sent out to the insureds address. To clarify by speeding I mean 10% above the stated limit, so doing 34mph in a 30 is classed as speeding.
I do not work in insurance or the Police so someone better addressed can advise, but this was in the paper about 2 months ago.
Personally I would steer clear of them but if needs must!