Check the identity of someone representing HMRC in person.
HMRC has published guidance on how to verify the identity of officers or collectors who visit you in person. If you are unsure of a visitor's identity, you should request their name and serial number from their identification card and call the specific verification telephone numbers provided in the guidance. These numbers are strictly for confirming the identity of a person present at your premises and should not be used for general tax enquiries.
Source: https://www.gov.uk/guidance/confirm-the-identity-of-someone-representing-hmrc. Summarised automatically from the official source — general information, not advice.