Have you changed address at any time before the alleged contravention and forgotten to update your V5C? This most likely reason for not finding out about a Parking Charge Notice (PCN) until a useless debt collector becomes involved.
Check your V5C. Many people update their drivers licence but either don’t realise or don’t remember to also update the vehicle registration.
If that’s the case, you need to urgently send a data rectification notice to the DPO at Horizon instructing them to update their records with your current address for service and to erase any old address they may hold for you. Not doing this is almost certainly going to get you a CCJ by default.
Otherwise, you can safely ignore Empira and any other debt collector. They are powerless to do anything except get the low-hanging fruit on the gullible tree to pay up out of ignorance and fear.
If you get a Letter of Claim (LoC) or an N1SDT Claim Form from the CNBC, come back and show us. Anything else is irrelevant.