VATREC14050 | Other Invoicing arrangements: Authenticated receipts: Claiming input tax against authenticated receipts
From HM Revenue & Customs · VAT Traders’ Records Manual
Where a trader cannot produce the relevant authenticated receipt to support a claim to input tax, the claim may still be allowed in the following circumstances:
satisfactory alternative evidence is available (Reg 29(1) of the VAT Regulations 1995 refers); or
a trader can show that reasonable efforts were made to secure an authenticated receipt from the supplier, and you are satisfied the claim is otherwise credible.