I don't know anyone who's applied for the same masters course as you're thinking, but I know several in other courses who stayed on or went back to do masters at Cambridge and many of them were accepted with 2.1.
They usually ask for a first from the students who did undergrad at other unis but it seems they're more accommodating for their own students.
I think the best thing to do for you is to go and discuss it with your DoS. He/she'll be the most crucial referee (and probably a supporter) when you're applying for postgraduate course.