I forgot to mention, apparently it is standard practice for debt collectors to send letters out three times. So after you've got in touch with them, don't be surprised that they send another letter to the same fellow again. They need to tick off the box in their "to-do" list.
I once rented a property where the previous tenants had done a runner leaving unpaid utilities bills. I had to write letter after letter, giving them the address of my local council (so that they could check that I'd been paying council tax etc and when I moved in). Eventually they gave up sending those stupid letters. Don't lose heart.