The MoT inspection manual for exterior and interior mirrors states that an obligatory mirror being slightly damaged or loose is a minor fault, but that an obligatory mirror not providing an adequate view to the rear is a major fault, so it may be that it will depend on the judgement of the tester as to whether the view is satisfactory. It may be worth replacing the mirror glass in order to pass the test and repairing the rest of the mirror at a later date.