The finance on my van has been disallowed as an expense how to change this

The Community has got you covered, @userjosephjamesmoore.

You can categorise this as a Travel expense, so you can claim it as an allowable expense for self-employment. 

1. Navigate to the Transactions menu.
2. Select Add Transaction.
3. Click Business if the transaction was for business, or pick Personal for personal. If the transaction was both, tap Split.
4. Choose Travel Expense as a Category.
5. Tick Save.

Once finished, let's pull up the Tax summary report. From there, you'll see that the transaction is now under the allowable expenses. The report will also display those transactions you've assigned under Travel expenses.

For your reference, please see this article: About SA103F Categories. 

With the suggested steps above, I still recommend consulting your accountant to assists you in re-categorising your transaction. This is to ensure that your books have accurate records and to avoid messing up your account. If you don't have one, use our Find an Accountant tool to look for an accounting professional near you.  

Also, I've added this helpful link to guide you in managing your transactions in QBSE: Categorise transactions in QuickBooks Self-Employed.

Feel free to post here again if you need further assistance in categorising your transactions in QuickBooks. I'm always around to help. Have a good one.