I've seen friends get fucked over by lack of guidance and/or support from their teachers and supervisors, and it is just crushing :( I dread to think what my dissertations would have been like without the amazingness that was our Head of Department and a couple of other lecturers (even though I was technically 'downgraded' to a different degree, but that's a tl;dr story).

This is one of those cases where I'm actually very glad that there is only 'pass' and 'merit' for MAs (mostly) and you're not obligated to give an overall mark. You've clearly worked hard for it and you didn't deserve this because someone couldn't be bothered to do their job (and the right thing).