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

HMRC internal manual

Aggregates Levy Guidance

Unjust enrichment: The undertaking

Once procedures have been agreed, a formal copy of the undertaking (Word 32KB) should be sent to the business for signature. But remember, before the undertaking is finally agreed we would expect businesses to have their arrangements and information prepared and to begin contacting customers as soon as the refund is made. Although we don’t expect you to check this, if there is any reason to believe that the business having signed the undertaking is unable to meet its obligations then you should ask to see details of the arrangements before agreeing to accept them. If there are any difficulties contact the Unit of Expertise in the first instance.

No refund must be made until the undertaking is received by you.