Hi Emily72.
I'm going to leave this one open to the wider community, as it's normally one i'd direct you to an accountant for. Formally, please speak to your accountant about what needs recording.
Effectively, there's a few different ways to account for this, and it often depends on whether you're VAT Registered, whether you've paid the VAT Up front on the vehicle, or if it's to be included as part of each month's payment.
You can create a yearly Bill, showing the total amount payable in this year, and then create one for each year. You could create One Bill with the amounts,and you pay it off bit-by-bit. With you having the long term liability, you may be able to just pay off the liability using the expense function. Does your accountant want you to show the "Expense" to the business as part of the monthly HP payments?