I had similar a few years ago on a 4/4. I ran a long jump lead from the battery neg to the starter mounting bolt to bypass the existing earths. Car started ok so it showed a bad earth. I cleaned the straps ( battery to chassis and starter to chassis) and cured it.
A second possible problem could be a sticking solenoid. Newer cars have one on the motor body itself rather than remote. Tapping it carefully with a wooden rod can free it. You then either get it cleaned up to stop sticking ( also check the wiring) or carry a stick! I did this years ago for a couple of days until I could clean the solenoid/motor.
