You 'did not receive the letter', if you know what I mean so you could not reply to it. As long as you don't mind going to court etc. you 'can' get away with it.
Pesonally I believe that if you know you have done wrong then fair 'nuff accept it. However when I think they're just money-making (dual carriageways etc etc, especially on the A1!) and hiding to catch you then you should mess them about. That way even if you do pay it costs them more in admin. If everyone did it then they'd stop ripping the motorist off even more!
I've had three letters and no points yet! Causes a bit of stress but saves money in the long run.
Check out
www.pepipoo.com
CheeRS and good luck.