First step - Edit your post to not reveal who was driving. "The driver" drove and you'll deal with the matter as the keeper.
Let's also see the notice you received - both pages. Redact personal info but leave all dates and times on there. Whilst people generally redact their VRM ('reg plate' - what I assume you mean by 'car number'), it might be worth leaving on the part that is different from your own, and telling us what it would read if it were accurate.
The fact that the notice has made its way to you suggests that it may just be a misprint on the notice. The notice is essentially an invoice - a typo doesn't automatically render it 'invalid', but let's establish the details.
So we can also explore other issues, can we see photos of the signage at the site, and a photo of where the car was parked?
