Hi Emma, 

Thanks for getting in touch. 

The easiest way to access this would be via an accounting documents report, as this will allow you to apply specific criteria to different record types, but it should also be possible to drill through to related credit notes from a contracts report using the Related Credit Notes drill through. I would recommend familiarising yourself with our Knowledge Base pages on Reporting for more information on this. 

Kind Regards, 
Workbooks Support