OK thank you.
At first Hazel was appending new comments to the existing comment but now it seems to be replacing the old comment with the new comment.
I am using colour tags as a revolving sort of notification system to let me know how long a file has before the next action is processed against it in the file is moved and stores elsewhere. I was using the comment fields to populate a new comment based on the colour tag to remind me of what state the file is lying in and wanted to make sure I could clear the comment and replace it with a new one that matches the colour state but now it seems to be replacing the comment entirely so I think I’m OK.