Your Discovery has what is referred to as a starter relief relay mounted on the frame rail close to the starter (passenger side)
This relay (see attached) is known to become corroded from exposure and causes intermittent starting problems.
The one photo shows the typical configuration of how its mounted on the frame rail and the second photo gives you a general idea of what it looks like.
Carefully remove all the cabling and clean up the connections real good and see if that aids in resolving your starting problems.
Another thing to check is you main chassis ground coming off your starting batteries. Make sure it is clean and tight.
Please let us know what you uncover.