300tdi intermittent starting problems

Ok. Weird one. 300tdi defender. Intermittent startimg issues.
Always starts like the battery is low. Starter motor is 12 months old. Battery is 3 days old (thought that was the problem).
The problem is this. Often when on a run when I stop it seems the battery is dead ish. Then if I wait for 10 minutes it fires up like normal. I have checked all earths. I have replaced the battery but no change...
Any thoughts?

Check alternator,

Then I'd say a new starter might be needed

usual simple checks first.

jump lead on engine to neg of battery term, see it it starts better. if so, it's an earth problem.