Have you ruled out ID theft?
I think it was maybe IDT the first time, but the debt guys aren't very co-operative when you start demanding proof that you are the one who owes the money - usually 'cos they don't have anything that would stand up and it is against the rules to badger the wrong person.
The second time out it was our Child Support Agency, who are beneath contempt.
This time it's just a company trying it on; they've been named and shamed many times on the TV and various consumer forums for pulling this kind of crap, but our hopeless government won't shut them down.
Unfortunately, the UK has so much overdue personal debt now that a whole industry has sprung up around selling it on to dodgy companies who then use various tactics to collect the money - from whomever they can.
They won't get anywhere with this one, but it is a pain in the arse.
