Just thought I'd post an update for those who might be in a similar situation.
My tutor emailed the faculty office on Sept 12th, when I got my results, and I heard back today. They upgraded my degree to first class degree and apologized! My average was 69 - there had been an error with calculating my third year grade as I split it over three years. The total number of weighted credits by year was more than half, so the secondary rule applied. As they initially calculated my average at 64, they didn't consider the secondary rule initially as it wasn't in the boundary for it to be applied.
I didn't need extenuating circumstances either, they just upgraded it without me having to file an appeal or anything. I'm just glad I was right - would be very embarassing if I'd finished my masters but calculated my grade wrong lol