Hi, personally I would send something like this:
Thank you so much for clearing my debt, I really was struggling, as you clearly know the situation everyone is in, your good deed has helped to ease my situation tremendously, I am now able to get a good night's sleep.
It was truly a very generous deed you did for me, even though I have never heard of you. If more people like yourselves would do the same for every other person who is struggling in this climate, what a place this would be. It makes a refreshing change for a bank to clear someone's problem instead of robbing us.
Again, many thanks for you purchasing and clearing the debt. I will deffinately think of you come Christmas time, thank you.
But hey, that's just me, guess it's my sense of humour.
Remember, they said they have purchased the debt