200tdi Disco Intermittent starting

I have replaced the starter & solenoid, Removed , cleaned and refitted the three earth straps, and I get 12 v dropping to 10 on the battery when the starter engages. The engine will turn about half a turn very slowly then stop.Voltaage stays at 10 v. I have also used a jump cable to run an additional earth from the battery to the starter casong. No good. Any ideas welcome