Just missed your last post. My suggestion would be:
"Dear Sir or Madam,
Reference : Unknown (Copy of your correspondence attached)
I've recently had to deal with a rogue direct debit being set up from my bank account, followed by your last letter reference xxxxxxx chasing up payment - for something I have not purchased.
I am unaware as to what this payment is supposed to be for. I would appreciate it if you could forward documentation supporting this debt to me at your earliest convenience - original, signed direct debit mandates, credit agreements, original invoice/payment paperwork or anything else appropriate.
Failure to do so will result in me taking matters further, which could in turn result in you being investigated for fraudulent activity.
I hope you co-operate with my simple request and that you can understand my confusion. Please keep communication in writing to eliminate any confusion.
Yours Sincerely,
Joe Bloggs"
Put it in snail mail with proof of postage. Keep a copy of everything. Don't worry about it, really. You'll probably even feel better after having effectively shouting these idiots down
And guess who spent far too long writing such letters in the past to banks/credit cards et al
