Help please.

I had a letter arrive through my letterbox recently from a company called 'Ruthbridge Limited'
They give some information, such as Their ref, Their Client, which is Cabot Financial Europe Limited, their Clients Ref and balance, which is 2022.04
It then says We have been instructed by the above named client to retrieve this outstanding balance in full.
Failure to respond to this notice within seven days may result in the following legal action:
- A county court form being issued
- A judgment being obtained, which will accrue costs and interest.
- A Bailiff's warrant may be served for seizure of assets.
- An attachment of earnings / Garnishee Order.
It then goes on to say I must contact them within seven days and payment must be made in full.

So I contacted them, having no idea what this debt is, the person in charge of my case wasn't there at the time, so I left my phone number.
Today He phoned, I asked him what the debt was for and he said that "I had been told before, so how do you intend to pay" this discussion went on for a little while, then He said "phone the police and get a crime reference number", I said "OK but won't they want to know what for?" he said "No, because I'll be phoning the police too" none of this made sense, how can I report a crime when I don't know when that crime is?
Anyway.. He then asked if I lived at an address he had, which I did, but quite some time ago...
He then said that I had a City Financial credit card at that address in 2001, and I had "done a bunk!"

I do need to explain something here, I left my home after a marital break-up, and lived in a small flat (the address he had) for about 3 months, then I moved back into the address I lived at previous to that to be with my children, (my ex-wife had moved away); I can't be certain that I didn't have a credit card, My mind was all over the place! but if I did have a credit card, how did I get it? I had just moved home, I was self-employed and on financial assistance, and surely on my application they would have needed my previous address?!?!?

I asked him to send me details of this debt,telling him that I'm not phoning the police yet, he said He wasn't going to send me any details whatsoever!
In all this time I haven't received any letters from City Financial, Cabot Financial, or from Ruthbridge ltd until now.

I'm at a lost what I should do now.

any advice would be welcome.
Many thanks.