Thank you for taking the time to reach out to us here on the Community page, @graham-hogg.
We're able to import bank transactions to QuickBooks Online (QBO) by preparing an Excel CSV file. However, importing purchase order to QBO isn't available as of the moment. To bring the details to QuickBooks, you have to input the transactions manually.
To create a purchase in QBO:
I understand the importance of being able to import purchase orders to QBO, @graham-hogg. I'll take note of this and ensure to send feedback to our product developers. This way, they'll be able to know and might consider the feature in the coming updates.
For additional reference about importing bank transactions using Excel CSV files, read these articles:
Post again if you have any other questions. I'm a post away to help. Have a good day!