Hi,
To be fair I didn't investigate any further. When I submit my mileage expenses for the month, I just split it into 2 lines, VAT recoverable and not. I've got two expense types set up in Travel & Accomodation and then I just allocate the appropriate amount to each type, see file attachment.
Shame it's not simpler but it's hardly a bind. I was advised to comment the mileage and amount in each transaction just for traceability. BTW the HMRC allowances change periodically, you can find the current allowances here:
https://www.gov.uk/government/publications/advisory-fuel-rates
