Should i not send the first letter back to them saying they probably have the wrong person?

Hi, a few days ago i got a letter from a crowd called 1st direct. They say that if i do not respond within ten days they will assume that i am the person they are after. I am aware of a debt from around six years ago but it is vague when it occurred due to my drinking very heavily at the time. How do i find out if this is indeed a debt belonging to myself? I have no desire to contact this 1st credit crowd having read about them here. What should be my first course of action? Ought I ignore this letter? I suspect they are fishing here, and i am fairly certain the debt is statute barred but as I said, I really can`t recall exact dates.