A list of recent examples of scams where fraudsters pretend to be from HMRC makes interesting reading:

https://www.gov.uk/government/publications/phishing-and-bogus-emails-hm-revenue-and-customs-examples/phishing-emails-and-bogus-contact-hm-revenue-and-customs-examples

As with all of our tax tips and web pages this information is necessarily summarised and of a general nature.  If you would like detailed specific advice please contact us.