First off, get yourself the proper bently manual from bav auto or somewhere. (Bav auto part number B390 - put that number in "search")
There are 2 things that could be wrong with it (Source: Bently manual), first the solenoid could be bad, or the start drive pinion or armature shaft could be fualty.
You'll simply need a new starter. Nothing you can really repair, bav auto part number SR443X, or go to an auto parts store and prob find it a little cheaper.