Checking the main engine to chassis earth would be a start ... costs nothing .... you could try connecting a jumper lead from the body of the starter to the negative terminal of your battery or a good chassis earth if that's not practical .... if that does nothing then move on to the next step. Good Luck
Check first the battery chassis connection and that to engine casing itself - usually flat braided high current cable.
If all ok there are two other possibilities. If you only hear a faint click, from the starter relay it could be this faulty or complete failure of the piggy back starter solenoid. You can check with a voltmeter whilst someone turns the key. If you hear a loud clunk this indicates the solenoid is trying to engage the starter pinion but not supplying current to motor underneath, or the starter motor itself has failed.
Most +8 owners carry spare starter relays. My S1 roadster started doing what you described. Pushing back in gear would solve it so I was going down the starter motor route but I thought I would check the earth first.
A jump lead worked so I removed the connection on the gearbox. Though outside it looked fine it was heavily corroded on the inside face of the connector.
I suspect moving the car rocked the earth lead to give a better connection.
