by Graeme Dunkley.
Thankyou Mary & Ben for your help, I think there is a ghostscript problem.
I run the diagnostic attached to the "Annotate PDF plug-in and it says, "The ghostscript path points to a non-existent file".
But when I checked with our hosting service, the value in the plug-in text is correct. He suggested I throw myself on the mercy of the moodle community to solve the problem.
I can see a tracker on a similar issue here: https://tracker.moodle.org/browse/MDL-42143 but I really don't understand it.
I've read through the conversation here: https://moodle.org/mod/forum/discuss.php?d=244304 but again i think i have a more deep-seated problem with ghost script.
Any further advice?