Letting agents are commonly pretty useless, we used one once who said he had not got any replies to letters demanding rent debt so that would be the end of the story. He knew where the guy worked, I went there and spoke to the ex-tenant (split from partner, hence moved out), very amicable conversation - in the end he actually paid. Another tenant of 14 years whom we helped in much the same way you have thereafter set a number of authorities onto us (brother said it was her way to deflect from debt to us - apart from irritation/tedium not a real problem) - we had considered her a friend because my wife knew her previous to the tenancy as our sons were of similar age and played together (although hers always was very difficult). Your situation is worse and the advice you already have points the way forward - why should you be out of pocket as a reward for help and good on you for quenching any urge to "let loose" with nastiness ? However, I would not go to a debt collector unless it is on a no-win-no-fee basis and that you can expect to get your money in full. Getting court decisions enforced is more difficult in the UK than in many other European countries, from my understanding of the systems in other places, because it is so much easier in the UK to "disappear" again and again, yet still be living a "normal" life within the country.