Thanks for the clarification.
I suggest you consider approaching a tracing agency run by ex-Police called http://www.bnhassociates.com - they have a high success rate and should be able to track the debtor's address.
Once you obtain the information i suggest you send a letter before action. The letter before action should outline why you are owed the amount in question and demand payment within 14 days, failing which court action will follow.
I have attached a template copy for your consideration.
Please send the letter via email and post when you get the address.
Hopefully the letter will compel a response from the debtor very quickly.
Assuming the debtor does not respond to the letter or a resolution is no met within 14 days from the date of the letter I suggest you consider commencing court proceedings using the link http://www.moneyclaim.gov.uk.
A user guide for the link to assist you with the court procedure is noted below:
If you win then once you have the CCJ from the court the defendant has 14 days to pay in full. If they do not then it gets registered with the credit agencies after 30 days. You can also enforce the CCJ with the county court bailiffs or transfer the debt to the High Court for a small additional fee given the total amount owed is at least £600 and you can use the high court enforcement officers who have greater powers than county court bailiffs. The transfer fee is added on to the debt and payable by the defendant.
I hope i have clarified the position, if not please let me know.