If the letter you got is genuine and gives details of the Court, then he must have been to Court and got a County Court Judgement against you. For him to do this, you should have been sent his Court claim and been given an opportunity to defend his claim in Court.
If you did receive the claim and just ignored it, then you are at fault and can probably do nothing. If you did not receive it then you may be able to get the Court to set aside the judgement on the grounds that you were not aware of the case and that if you had been you would have defended it and the outcome would very likely have been different.
Did he have your address, because any Court claim has to be sent to you at your last known address?
You really need some local advice on this - CAB or other free local money advice service, or possibly phone CCCS.