thanks again for assisting. i have resolved this now since i realised that the php file i use to stream the files did not support partial content ;)
i thought that nginx had some kind of built in 'magical' support for that which meant that my application didn't need to handle it. however, that is not the case and once i added in a PHP class to handle the partial requests, the files now seek correctly. i am seeing some zero buffer errors in the nginx log for large files, which i haven't debugged / resolved yet - but the major issue is resolved now.