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.
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