blowing 15 amp fuse in starter motor circuit mercedes?

Blowing fuse no52 in mercedes c220cdi 52reg.Fuse diagram says no52 is start enable.I changed 15amp fuse then starts ok for a few days.
today blown fuse 15 amp 3 times,so I put 30 amp in to get me home ,it started ok but I dont want to run with 30 amp in.I dont know if there is a starter relay in circuit causing the problem,the diagram does not list the relays.
Good possibility it's the starter motor ???
Please any help greatly appreciated.

tomg
April 2009