Can you run a report to show all customers which have had a credit note processed in a selected period, showing the customer and relevant credit value by transaction?

Yes, this is possible in QuickBooks Online (QBO). I can walk you through the steps on how to generate this report, @richard-diversef.

I suggest running a Transaction List by Customer report. You can then customise it to show credit notes.

Here's how:

1. Go to the Reports menu.
2. Type in Transaction List by Customer on the search box.
3. Click the Customise button.
4. Set the Report period you need.
5. Move to the Filter section, then set the Transaction Type to Credit Note.
6. Hit Run report.

You'll then see the report you need at this point, with credit notes sorted per customer. 

Keep this article for future reference: Customise reports in QuickBooks Online. It has a list of sample custom reports, with the steps on how to generate them.

 Post a reply if you have other questions about reports in QBO. I'll be sure to get back to you. Enjoy the rest of the day!