Aidan,
Centric CRM ships with JasperReports library which is also freely available at
http://jasperreports.sourceforge.net/
If you have downloaded the source version of Centric CRM, then you should have the report xml files under src/jasper_reports directory. You can modify the reports using any available jasper reports editor. The reports that ship with Centric are tightly integrated with the application itself. You should be able to modify the reports and view the results when you deploy the application.
But if you wish to write your own reports, then you might need to understand how reports are integrated with Centric. A quick starting point would be the documentation available on the Centric website under the 'Order' page. There is a very well documented tutorial to get you started with writing your own custom jasper reports.
Hope this information helps!
-Ananth