I think this may be one of those cases where the law will not help, as you will have no proof that you gave the money or the purpose that you intended it to be used for, if you have proof you gave it, your ex boyfriend may easily say you were repaying a loan from him or some other reason. Your best plan would probably be to ask a local Solicitor to write to your ex boyfriend stating you requires the return of the £10K loan, and if he does not have the cash suggest he sells the property if he still has it (you do not say) and hope he feels a liability, sometimes letters from solicitors can help.