surfinaswain, it is quite likely you are in serious trouble. In most modern diesel cars, the engine management system starts to prepare for the engine being started as soon as you blip your remote control to open the car doors. This means that the fuel pump starts to transmit fuel from the tank towards the engine. Petrol, being more processed than diesel, does not have sufficient lubrication for the fuel system and engine. You might need more than just the tank being drained. It might mean an engine stripdown. As Ron Burgundy noted, it depends on how much you put in, how much diesel was already there and at what point you noticed that you had 'committed an oops'.
Best of luck...