I didn't do official mocks, just timed essays and past exam papers in class, and for half of my subjects they were pretty accurate, but for the other half they weren't. I always got high As in my sociology essays, which turned out to be accurate, and in past papers for French I always got As, although I got higher As in the real exams. English lit was really erratic; I got a mixture of As, Bs, Cs and even Ds in my essays and got Bs in the real exams. History was pretty inaccurate too; I always got As in unit 1 and got a B in the real exam and I always got As and Bs in Unit 2 and got a D in the real exam! I'd agree with what everyone else has said- use mocks as a guide, but don't take them as gospel.