Have a read
here and
here.
This company is a Private parking company and what you have received is similar to so many others. You will have a fight on your hands as (from what you will read in one of the above threads) AS Securi T are one of very few who may try to issue County Court proceedings to get what they see as 'their' money. Keep copies of letters however and send them by Recorded Delivery so that you have proof of receipt and even if they are daft enough to go that far they can be beaten.
Write back (in short but polite terms!) and inform them that you have received a letter stating that 'you' owe them money, however they seem to have not replied to your original question as to why they believed that you owed them money as you have never seen the PCN which they claim to have issued, and also that it is by no means certain that you were the driver on the day, and therefore may not be responsible for agreeing to any contract which they may believe exists. Ask them once again for some evidence as to why they believe that it was you rather than anybody else who made any agreement to pay the amount stated.
You also need to add the following :-
I note your threat to send a representative to my property to discuss this matter. Please be advised that I will
only communicate with you in writing.
Furthermore, should it be your intention to ignore my request and to send a representative anyway, please be advised that there is only an implied license under English Common Law for people to be able to visit me on my property without express permission (the postman and people asking for directions etc [Armstrong v. Sheppard and Short Ltd [1959] 2 Q.B. per Lord Evershed M.R.]). Please therefore take note that I revoke license under Common Law for you, or your representatives to visit me at my property and if you do so, then you will be liable to damages for a tort of trespass and action will be taken, including but not limited to, police attendance.
They will no doubt ignore this letter also as it sounds as though they cannot put up any sort of argument so continue with threats instead.