Thanks for your question.
You would first have to get a judgment on the debt by issuing proceedings in county court .This is fairly straightforward assuming you have documentary evidence regarding your contract with him, or he does not dispute this and that it is a contractual term that you should have been paid by now.
After you have obtained judgement on the debt you can apply for a charging order against a property he owns. If he does not own the barn then you will not be able to get a charge against this, but if he owns other property - even better, without a mortgage, then you can apply against this property.
You could also then apply for an order for sale on it.
I would considering instructing a local solicitor to run the litigation for you if the sum owed is substantial and certainly checking the Land Registry for the titles to see what he does actually own by using the online "Land Registry Find A Property" service but it is not available on Sundays unfortunately.
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