HMRC have updated from 1 December the advisory mileage rate used for recovering VAT on fuel costs within mileage.
(a) use our PSC spreadsheet
(b) are vat registered
(c) not on flat rate
(d) claiming mileage
Then you need to make a change to the rates you use – instructions are here https://www.whitefieldtax.co.uk/update-to-hmrc-advisory-mileage-rates-2/
This doesn’t effect non vat registered clients, or those on flat rate.