2017 vauxhall insignia 2l Sri nav cdti auto

Help please

I have an intermittent engine overheat problem . It’s very random but every couple of weeks or odd days when the engine is hot if I turn it off to get Petrol , get a pint of milk it then won’t start up . Engine won’t crank and like it has a flat battery . Engine management light comes on amber..

problem started after a long journey when engine overheated due to small splt in coolant reservoir and loss of fluid . This was replaced but NOT the sensor.

The temp gauge on dashboard doesn’t go above 90 . After everything g has cooled down it will start again and runs fine . Garage said starter motor issue and charged £800 but problem still ongoing . They have had it back in and cannot replicate fault or find anything g wrong .
Last few times it’s happened I have undone the coolant cap and after 5 mins once cooled down the engine will start again . To me it seems like some sort of sensor or thermostat type problem stopping engine from starting . Any similar issues or ideas ?