I've solved it; or rather my boss did.
There was a problem with the way I'd formatted the report, I'd deleted the header row so I was putting my fields in the wrong row. That meant it would only ever show one row. It looked alright at first glance so it took a while to spot which is why he didn't see it the first time he checked.
Thanks for your help.
How to post a question to get the most help http://www.sqlservercentral.com/articles/Best+Practices/61537