When you say you “ignored” previous letters, are you sure that you did not receive either a Letter of Claim or an N1SDT Claim form from the CNBC?
It is unusual to receive all PCN and debt demand correspondence but then not to receive both an LoC and a Claim form. You need to call the CNBC and get them to email you the details of the claim to include the claim number, the address it was sent to and the Particulars of Claim.
If the claim was issued correctly to your current address, you have very little chance to recover the funds that you are now intending to pay to prevent the CCJ from remaining on your credit record. Is there any possibility that the PPC hold two addresses for you? Did you ever move and fail to update the address on your V5C logbook?
You do not “just fill out a form” to file for a set aside. If the claim was sent to your correct address, how are you going to evidence that you never received it?. If you can evidence that it was either sent to an old or the wrong address, then you have grounds for a mandatory set aside under CPR13.2.
If you can convince a judge that you never received a claim form even though it was sent to the correct address, a set aside under CPR13.2 would be possible but good luck in proving you never received it. The only other option would be to request a set aside under CPR13.3 which is discretionary. For example, if you believe you had a good chance of successfully defending the claim.
It will cost £303 to apply for a set aside unless you could convince the claimant to agree to an uncontested one which would cost only £119, assuming they can be bothered to assist you once they’ve received the claim money from you.