Thanks for your question.
I'm afraid there is no magic bullet in these situations. You need to trace him (obviously) and sue him.
Did you obtain any references for the tenant before they moved in? if so, write to them and see if they have an address, if you used an agency contact them as well. Alternatively you could speak to an enquiry agent (you can google for a list of them in your area) to do some investigative work for you - make sure you agree a price beforehand.
As far as suing and keeping your costs down, www.moneyclaim.gov.uk really is the cheapest way of doing this and was designed for situation in which persons who are owed relatively small amounts of money where it is not cost effective to instruct a solicitor.
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