Putting petrol in a diesel van can cause catastrophic damage. The petrol will effectively reduce lubrication and cause metal parts to rub together and become damaged. To prevent this from happening, you’ll need to have the tank drained and entire fuel system flushed. Do not start the engine as this will circulate the petrol into the system.

If you don’t want to pay for a drain and flush, you could try filling up with superdiesel (which has more lubricity additives than standard diesel) and add a lubricity additive to the tank. If you keep the tank above half full for 1000 miles, you might get away with it. However, to be honest, it’s not a risk I’d take.