Hi, it used to be that the notice had to be issued within 28 dyas of the offence, as it was unreasonable for you to be aware of the offence you had comitted at that time.
however , I've looked on the web and can no longer find details of the 28 day time limit.
I've put a link on to a police website which gives details of faq's. It seems to be if you can't remember then the registered keeper will be called to court.
I would get a quick phone call to your solicitor or legal advice line on your insurance if you have it, just to check, it might save you 3 points and �60, more if you contest it, you are unlikely to get less if you contest it.
http://www.devon-cornwall.police.uk/v3/roadsaf e/fxpenfaq/index.htm