Possible fraudulent letter

I got a letter supposedly from HM Revenue & Customs re Amount still overdue re 'PAYE tax and National Insurance payments'. The company closed several years ago and the Accountant then and now states that all accounts are in order and no payment outstanding. I have been trying to get in touch with HMRC to see if this is a fraudulent letter but cannot find a phone number or email address for anyone who can help. Anyone I have spoken with (after holding on the line for up to 1 hour) says to go to hmrc.gov.uk and get details there! I have gone to hmrc.gov.uk but have been unsuccessful in getting correct contact details. What should I do? Thanks

Hi
You can view your PAYE account balance online if you still have access to your government gateway log in credentials.
If not, you can contact the Employer Helpline on 0300 200 3200 and an adviser will be able to confirm if your Employer PAYE account has any underpayment of tax and national insurance.
Alternatively, you can contact our Debt Management department on 0300 200 3814.
They pursue any outstanding PAYE liabilities and will be able to confirm if your company has any overdue balance.  
Thankyou.