Were you doing 80 before the sign or after? Unfortunately you are not allowed to speed up before the signs afaik so he may have you on a technicality. Also if they were 100 meters or so past the sign and you sped up just before/as you passed the sign I'd definately argue that he could not tell you where speeding 'before' the sign..

At the least consult a lawyer, most will give one free consult..

cheers

Brett