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

HMRC internal manual

Claimant Compliance Manual

Opening and Working Enquiries: Information notices - inability to comply

If the customer(s) informs you that they cannot obtain the information, or says that the evidence you have specified does not exist, and you are satisfied that the explanation is genuine, you should disregard the relevant items for the purposes of taking penalty action.

If you are not satisfied with the explanations you should write to the customer(s) telling them so and explaining that you will now be taking further action.