Same problem on my Defender
Hi! I run my Defender in Romania frequently on very wet andor dusty tracks. The starter motors only last 9 to 18 months, especially the Marelli Magneti ones! I change them often. Lucky for me I can still get starter motors repaired in Romania for about £40 a time - cheaper than replacements.
Sometimes the problem starts after the following series of events:
1. Dunking in water or long drive in heavy rain, followed by....
2. Getting the engine really hot (heavy load/steep hills), followed by........
3. Switching the engine off too quickly, rather than leaving it to tick over for a minute...
Temporary solutions include the following:
A. Often it will start after trying the key 10 times or more.
B. Holding a long steel bar to the starter motor where it enters the bell housing, and whacking the other end of the bar with a large hammer. If the bendix or solenoid have stuck, this will free them!
C. If I manage to re-start the engine after the starter has stuck, next time before I turn the engine off I "blip" the starter with the engine running. This seems to dislodge any tendency for the starter bendix/solenoid to stick.
Note that temporary solutions A, B & C don't do the starter motor much good, and a starter motor replacement will soon be needed anyway! But they might keep you going for a few more days - even a few weeks!
In Romania I always carry a spare starter motor and water pump in the spares box. Both components fail regularly and I am stuffed with a dead starter or a broken water pump!
Good luck!
Colin