This part of GOV.UK is being rebuilt – find out what beta means

HMRC internal manual

International Manual

Double Taxation applications and claims: Repayment claims from non-residents: Enquiries: When do you have to open an enquiry

If you need some additional information from the claimant, or his tax adviser, you must give notice in writing to the claimant of your intention to enquire into the claim. This applies even if the claimant has used an old style form that does not give all the information that we ask for in the latest version of the form.

You cannot simply contact the claimant or the agent, even by telephone, to ask for the information you need until you have given notice to the claimant, in writing, of your intention to enquire into the claim.