There can be some confusion where the customer's expectation is you treat all provisions of fuel card to employees the same, regardless if this is for a company provided car or a privately owned one.

The rules are very clear for company cars but the approach you take for privately owned cars is very different.

With reference to HMRC's CWG5 Guide, fuel payments in a privately owned cars are shown as follows

  So simply enter the amount spent on fuel in Section M (subject to Class1A NIC) either manually or via importing.