At this stage, you can only realistically get some money back in one of two ways - either by him agreeing to voluntarily pay you back, or by you pursuing him through the courts for it.
Assuming he does not pay anything himself, then your only option is to go via the County Court. You would initially have to make a claim and you will have to pay the court fees for doing so (although there are sometimes you can get discounts if you are on low income).
The court then considers your claim and if they are satisfied you have a good case they can issue a court judgment in your favour. You can then use that judgment to pursue him further for what you are owed by enforcing it against him - you can get things like court bailiffs to try and get some of his property to pay off the debt, you can ask his employer to automatically deduct money from his wages to pay you off and so on.
The issue is that none of this guarantees you will get anything back be cause he can have no assets or avoid these requests and force you to go further and waste more time and money finding a method which works.
Also you have no address for him and whilst you can use his last known address, as you can imagine if he did not know about the claim then it is unlikely he would do anything in order to try and settle it.
So there are options, but far from guaranteed to work and the problem is that they are all you have available to you so they will not be without their own risks.
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