Is it possible to create a report to show all outstanding (unpaid) direct debit payments?

Yes, that's possible, phil13. 

We can customise the Transaction List by Date Report. Let me show you the easy steps:

1. On the Reports page, search Transaction List by Date.
2. At the upper-right corner, click Customise. 
3. To show the Unpaid status of your transactions, click Rows/Column. 
4. Then, check A/P Paid for Suppliers. For Customers select A/R Paid.  
5. Select Filter to choose Direct Debit as the Payment Method. 

You can check out this article for more information on how to customise reports. 

I'll be here if you need anything else. Have a great day!