Just experienced the same problem on my 1990 Astra. Battery OK, connections to battery checked, headlights working fine, but starter wouldn't spin, solenoid didn't click, ignition lights working intermittently or not at all. Fault was the copper braided strap connecting the engine block to the body, on mine from a stud on the gearbox to the n/side chassis. Although it looked superficially OK, it was corroded and just bending it caused it to disintegrate. Replacing it cured the problem instantly.