Near Side Earth/Chassis bolt.
Note the two black earth cables, they go to the Off Side Chassis bolt. The earth bolt must be in good condition or the two chassis cables could burn out the loom going to the Off Side!
This earth braid is very important, as at the moment, I cannot find another earth cable to the battery. The battery earth cable is attached to the chassis on the battery side. So the earth path for the starter Motor is through the chassis from one side to the other, or via the two cables mentioned. Quite a long path. So to repeat, the chassis bolts must be kept in a clean condition. If these are in poor condition then the starter and Glow plugs will exhibit problems.
The "Gunk" around the bolt is grease I sprayed on to ensure no rust causes problems.