What must you do if put half a tank of unleaded petrol in your full diesel car?

Do not start the car. Putting unleaded petrol in a diesel car is serious. The petrol won't ignite properly in the diesel engine and could cause significant damage.

Immediately call a recovery service or roadside assistance. They will be best equipped to deal with the situation, which likely involves draining the tank and potentially flushing the fuel system. Don't attempt to drive the car at all.