Hi Davo,
Thank you, you have helped me find and fix the problem. There is a 'local' plugin in our install that had a leading whitespace in one of its files. I have removed it and all is well now. The plugin is a custom one, not available in the Moodle.org plugins database. I will get in touch with the provider to inform them, as other Moodle users may be affected (those who use PowerSchool as their SIS).
Cheers,
Fred