OK, so your earth strap to the engine has broken I would guess.
Could be corroded at one end or other too.
High resistance connection dropping the voltage when the current goes up, i.e. cranking & glow plugs.
See if you can figure out what size the battery cables are I may have some 16mm tri in the unit, that would do, if they are 25, try Canoe for a m or so.
Some round ring crimp lugs and you're done.
I would not try to rely on 6491x type wire for long if that was what you had to fit for an engine earth, vibration and all that!
Glad you got is sorted mate, as I say if it is 16mm you want, I'm sure I can send a m or so up.
I'll check the next time I'm in the unit, Mon or Tue probably.
HTH