If you have received a Judgment for Claimant out of the blue, then you have a (default) CCJ. This happens a lot as drivers fail to reply in time or the form has been sent to the wrong address. Once you get it, you have a reasonable period of time (about 28 days) to apply for a set-aside and to have your case reheard.
Issue 1: Failing to reply on time
Unless there is a very good reason why you could not reply in time e.g. hospital or other medical condition, then a set-aside is unlikely to be granted. It would be a waste of the £255 fee though people who rely on a clean credit record should at least try as courts have some limited discretion on the matter.
Issue 2: Wrong address
This comes down to whose fault. Parking companies appear to be sending out letters to find out if people have moved and then sending claims in the hope of getting a default CCJ. The view is that if the Keeper has not updated their address with the DVLA, there is a chance the court may decide not to grant the set-aside hearing and they get money without having to justify it.
As well as this, most people do not know how to get a set-aside and pay out of fear and ignorance.
The National Debt Helpline (0808 808 4000) has a guide to setting aside default judgements. Details on in the link below.