If you want to do it the legal way:

Sent a letter to his address with your grievance(s), the solution to them and a reasonable date the solution must be executed by. Also go on to explain that the letter is a "letter before action" and that if the solution isn't met by the specified date litigation will follow shortly afterward. Send this letter by recorded delivery and ensure you keep the tracking details.

If he doesn't settle the outstanding balance then go to https://www.moneyclaim.gov.uk/csmco2/index.jsp and start a small claim with HMCS.

Several things to be aware of: If you don't think he has the money to actually pay you back, then it's probably going to be a waste of time pursuing the debt in this manner - no matter how many judgments you may have, if he hasn't got the assets then it is somewhat pointless. Secondly, on the assumption the defendant is an individual: If the claim is disputed then you will have to travel to his local county court for the hearing, he will not have to come to you.