It seems likely that you have cooked the solenoid wire, the wire that runs from the ignition switch to the starter, get a length of wire and take straight from the battery side of the solenoid to the terminal onto which the ignition feed goes, even a slightly duff connection here will do what you want.
I did a escort diesel a few months back, exactly the same problem, the guy had bought new starter and battery, and was getting 12V, so was confused, this dropped to 7 volts under load, ad the bodged on crimp connector eye terminal has serious high resistance.
One soldered connector later, it started and has done so ever since, he reports it's never been that good beofre!!
tabetha