Hi all,
I wanted to update this thread, Richard is one of my engineers and he is on holiday at the moment.
I took on the case and using the xhprof/moodle profile tool I identified that there was a PDF Submission / PDF Feedback module being used on the mod assignment which was the source of the problem.
Removal of this 'pdf' directory in both cases resulted in immediate performance improvement from 60 secs down to 3 secs.
I then installed the latest version of both from https://moodle.org/plugins/view.php?plugin=assignsubmission_pdf & https://moodle.org/plugins/view.php?plugin=assignfeedback_pdf and all is still well.
I suspect this problem for us was due to the module not being updated when we updated from Moodle 2.5 to 2.6.
Not sure whether it will help anyone else but thought I'd share nonetheless.
Cheers,
Kim