Final (and first) demand for unpaid class 2 NI contributions

My wife today received a letter from HMCE claiming that she owes £509.60. Here is a transcript of the letter.

"Dear Sir/Madam

Please pay £509.60

Our records show that you have not paid the above Class 2 National Insurance contributions.

As you have not contacted us or paid what you owe we will now send your debt to a debt collection agency to collect it for us.

You have a final chance to avoid this happening by making full payment now."

[Details on how to pay etc.]

"Act now to stop a debt collection agency contacting you for payment.

Yours Faithfully

Officer of Revenue and Customs"

[Unsigned]

The above amount equates to 182 weeks (3 and a half years) of Class 2 payments at £2.80 per week.

First off, this letter is the first correspondence of any kind on this matter that HMRC have sent her. Why then is it threatening debt collection?

Secondly, my wife was self-employed for a sum total of 10 weeks, not 3 1/2 years, and notified HMRC that she is no longer self-employed.

Furthermore, her self-employment started 2 and a half years ago (April 2014) and ended June 2014, so why does this demand for payment go back even further than this? The statement on the reverse of the letter claims that she was self-employed in year ending 05-04-2011!?

Can anyone provide any advice, other than simply contacting HMRC NI dept. on this on how to deal with this. A standardised letter would be helpful. My wife is currently working (as an employee, i.e. on PAYE) long hours and will not have the opportunity to deal with this personally until the end of the week.

I would also be interested to know if anyone else has had similar problems and if measures can be taken to deal with such heavy-handedness, i.e. final demand with no previous contact or notifications.