My Disco 200 TDi has a problem in that occasionally the starter does absolutely nothing when the key is turned, but other times it works fine. The battery is fine. It is likely to be the brushes in the motor or a problem with the solonoid?
Try the small switch wire that runs to the starter, you may have to remove the heat shield to get at it. Mine did this and I found this wire was loose at the starter.
Another problem is the fused loom just aft of the battery where it runs over the inner wing but this will also have some other corresponding circuit failure at the same time.
