Hello Davo,
I know this is a very old thread but perhaps you can help us with this old add-on that you did.
I don't know if you have found a similar problem but when we use the drill down report we cannot export to ods or excel, files are empty.
We are going to move to Moodle 2.6 soon so if there is not a solution, forget this message. If you faced a similar problem and you had a solution we will very happy to hear good news from you.
Thank you for your addon and your help.