That looks like a 3rd letter rather than the original ntk or a reminder so potentially 2 missing letters. Do you have grounds to appeal, e.g, car broke down, medical emergency that prevented the driver leaving or did the driver simply over stay / not pay? If the former you might get them to consider a late appeal, if the latter you've probably missed your opportunity to appeal and will have to wait out the debt collectors letters until the inevitable court claim comes through. It would be worth sending them a subject access request so you at least have a copy of the original PCN which may open up avenues of defence should it get to a court claim.
I'm assuming you haven't changed address recently and this is down to Royal Fail? No possibility of someone intercepting these letter at your address, deliberately or inadvertantly? Saw a case on another forum recently where the keeper was incandescent they had received a CCJ in default, turned out his partner was intercepting the mail hoping it would go away.
