I have a Private plate and i have been fined twice and at a cost of £60 per time on the zed. But i have to say that both times they were stroppy coppers. I have had far more coppers been behind me, in front of me,etc and they havnt bothered. Its a risk you take really!
On another note, my wifes car has had J4NET on her car for 4 years and not once has she been pulled over.