Hi Robert
Create the bill as usual payable to the supplier*
Great a new bank account [loan account]
Pay the bill using the loan account.
Class the monthly repayments as transfers between your bank and the loan account.
Adjust each month/quarter/year [your choice really] for the interest.
*Vehicles should be listed as an asset on the balance sheet and depreciated appropriately. Ensure you're aware of the rules surrounding capital allowances [i.e. the tax treatment of the purchase] and the potential benefit in kind issues you may have [if LTD]. A couple of links below:
https://www.gov.uk/capital-allowances/overview
https://www.gov.uk/expenses-and-benefits-company-vans/overview