Its a scam!
If you receive an unexpected phone call or email from HMRC, then be aware of the possibility that it might be a FRAUD!
In the last week, two of my clients have been in touch with me. Both had received phone calls to Stonehaven land lines - supposedly from HMRC. Once was quite upset by the call. The tone was threatening - you need to pay NOW or legal action will follow. Of course, neither of these clients owe anything to HMRC - one has just retired and doesn't even need to do tax returns any more.
Some clients do have arrears of tax, and HMRC does make debt collection phone calls, and nowadays it also uses external debt collectors. However if you have any doubt at all about who you are speaking to, ask them to put it in writing, or tell them you will call them back. And call back on the tax helpline (or debt managment line). Don't call back on the number they give you, and if you are phoning straight away make sure you call back on a different phone line.
Fraudulent calls and emails need to be reported. It probably will make no difference in your particualr case, but it all adds to the pressure for action to be taken to stop these calls. See the Action Fraud website