The answer seems to have been staring me in the face. "core_grading/invalid_method_name" We were using Marcus Green's BTEC marking method on Moodle 2.4 however when moving to 3.0 we had not installed the grading method.
Adding the courses and assignments with the method set to "Simple Direct Marking" rectified the problem.
Now to install the grading method within 3.0 and see if it fixes the error.
Will update the post after testing.
Carl