thanks for you reply and great advice - I appreciate the time you take to help us folks out.
So, on closer inspection the conversion is actually working and correctly displays the PDF in the grading screen. However, if no submission has been made by a student, it shows the error message regarding corruption or unsupported format.
The message in the notifications page still shows "The version of unoconv you have installed is not supported."
So whilst it is not stopping us doing anything, it appears there's at least a couple of glitches somewhere that have revealed themselves since updating to Moodle 3.10.1+ (Build: 20210211), which I hope will resolve with further updates as they become available.