DO NOT communicate with a useless debt collector... EVER! All they can do is try and persuade the low-hanging fruit on the gullible tree to pay up out of ignorance and fear. Debt collectors are powerless and should be ignored.
If this is the first you are hearing about this, the most likely explanation is teethe V5C registration document was not updated with the Keepers current address for service at the time. You need to check that the V5C has been updated with the current address. If it hasn't, you can do it online. However, that will not help you now.
If the address was not correct at the time, then you MUST send a data rectification notice to the DPO of (not so) Smart Parking Ltd. You send it to dpo@smartparking.com and you also CC in yourself. Your DRN must instruct the DPO to update their records with your current address for service and to erase your old address. The highlighted words are there for a reason, so use them. That's assuming 'you' are the Keeper.
Aside from the, all you can do for now is ignore all debt recovery letters and wait for a Letter of Claim (LoC) to arrive.
If you confer that the V5C address was not correct at the time, let us know and we can advise on a formal complaint to (not so) Smart Parking.