Incomplete VAT records

I am facing a bit of issue, I have a client with incomplete records and I need to amend a VAT return in past year. But I have incomplete records, this is a retail company. Is there any HMRC guidance which may help me in this scenario ?

Hi,
If you have discovered an error on your client's VAT return then you can correct this as per the guidance below:
4. Correcting VAT errors on a return already submitted
However you do need to have accurate records to support this correction.
Please see the guidance regarding the record keeping requirements:
2. Record-keeping rules for all VAT-registered businesses.
Thank you.