HazyJane raised some important questions here. Additionally, was the dissertation second-marked? If it was and the marks, as HazyJane further remarks, were fairly consistent, then that should be fine. I think, Masters' dissertation should be double-marked, and an agreed mark reached by the two markers (either 2 internals; or 1 internal and 1 external) before the external programme examiner validates the grades, and the students' grade officially made public.