Mortimer Clarke

Hi, I received a letter from Mortimer Clarke a few months ago (June I think) saying I owed their client ME III Ltd £1687.16. I had no idea who this ME III was nor had I had any communication from them. I wrote to Mortimer Clarke saying I had no idea of this client of theirs and could they provide me with copies of any agreements relating to this debt. I had a letter back saying that any further action was suspended while they awaited the documentation. I heard nothing until another letter arrived from M.C on October 7 2017. The letter is dated 2 October 2017. In it they say that the debt relates to a hire purchase agreement with Black Horse Ltd entered into on 25/09/2006, terminated on 05/06/2013. The debt was then assigned to our client ME III Ltd on 01/10/2015. They have included the original agreement, Statement of Account & Default Notice. They then say "We can confirm the balance on your account is £1687.16 and is valid until 02/10/2017. My question is this debt still enforceable if it was only valid to 02/10/2017 (which was also the date of the letter) & bearing in mind I received it in the post on 07/10/2017? I also suffer with anxiety & depression & am on medication from my GP for it so really hope this can be sorted. Sorry for the long post!