Sorry I've taken so long to reply but I hope the following is of help.
I've now had a total of five letters from these jokers and one from their tame solicitors, Lawsmiths (same address). These have arrived over two and a half months.
Careful reading of the text makes me think that they are computer-generated - some contain minor factual errors in my case but could apply to others in similar situations.
They are all signed by Carly Shedlock (or it could be Shylock) who has given herself the title of Account Manager, each one more threatening than the previous.
The last one