If a judgment has been obtained against you without your knowledge you can apply to the Court to have it set aside.
You should contact the Court as soon as you learn of the judgment and obtain an "application form" (Form N244) on which you must set out the reasons for wanting to set aside the judgment You will need to support this with a "statement of truth" and pay a fee of £65.00
If enforcement proceedings have been issued against you you will also need to apply for a "stay" or "suspension" of those proceedings until the Court can deal with your application to set aside the judgment at a hearing.
If you did not receive any of the Court papers - because the papers have not been property served e.g. you have moved address - you are entitled to have the judgment set aside as of right.
If there is some other reason why you want the judgment set aside the Judge will consider your request at a hearing and may or may not agree to the judgment being set aside. Before doing so a Judge will want to be satisfied that you have an arguable defence to the claim.
If the Judge sets aside the judgment you will be required to send to the Court a written Defence setting out the reasons why you do not accept the judgment that has been made against you. The Judge may also impose other conditions such as the payment of a sum of money into Court - in return for allowing you to set aside the judgment.