I thought that I had it with this hint from https://stackoverflow.com/questions/58451528/parenthesis-in-sqlite-when-combining-union-and-except-statements

However; even with reformatting using what is mentioned in that post, and performing an intersection on a selection from (a union of exceptions), despite it running fine in my terminal - I'm getting the same 'you've used parenthesis incorrectly' error on moodle. I'm assuming this one may be because of the union within brackets, again...

Sadly it didn't solve it for me, but feels like it's getting closer anyway (assuming the moodle checker is correctly operating), but hopefully it helps someone else get theirs over the mark.