Wow, people still reply to the notice they get after they get caught speeding?
Not recommending that people do this but the police have a maximum of 14 days to get the notice to you but they don't send it by registered post so they have no way of proving that you got it. They will send you a 2nd notice about 4 weeks after the first at which point you could ask your solicitor to contact them with a letter stating the rules and asking them for proof that the first(and on time) notice was delivered(not sent)
Never done it myself mind as I've never been prosecuted for speeding.
I fell foul of this trap. Held a clean licence and I never received their initial notification letters and was shocked when they said it it was sent but not recorded/signed for. Nevertheless I was speeding 38 in a 30mph with a mobile camera (sorry about that) but knew nothing about it until a court summons arrived!! The offence went to court in July 2017 but I was speeding in Jan2017. I pleaded guilty and wrote a letter to say the letters didn't arrive and thus at the very least I should be fined the punishment at the time of the offence. they ignored this and fined me £480 plus costs and three points under the new regulations which came in late april. I wrote to the court to complain and received no reply once again. pretty horrible for a 38mph offence.
Sorry it is too late now but you need to ask them for proof that the letter was delivered and not sent and it should never go anywhere near a court. I 'know someone that have used this many time's but not me!
You need two things, know what to do and a either a solicitor that knows the law, you'd be surprised how many don't. Luckily I have a friend that done it for me. It is just a standard letter as in one you might send to your bank to claim for PPI or bank charges. It really is easy but sorry you got done.