Even if hmrc were carrying out an investigation they'd accept mileage logs as long as you can back up the reasons for yiur journeys. They'd never ask for petrol receiptsIt doesn't prove the mileage but it proves the fuel was bought to cover the mileage. Theoretically, HMRC can ask you for proof of your business journeys although they are extremely unlikely to do so unless it looks like you're taking the piss. If you have dodgy claims then they might make an assessment on your behalf and, predictably, you're unlikely to come out of that well. The onus is on you so perhaps your company are just being cautious?