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HMRC internal manual

VAT Refunds

Unjust enrichment: Reimbursement: How soon after the undertaking has been signed should repayments be made

We would normally expect the claimant to have the scheme ready to implement when we receive the signed undertaking and to have begun contacting customers straight away, rather than waiting until well into the 90 day period before doing this.

A late start to refunding the money without good cause will not be seen as a valid reason for extending the 90 days refund limit.