Like others, I put a 5 litres or so of petrol in once before realising, I filled it up with diesel then just ran the car, I could feel the difference but it ran ok, did another diesel top up later. Car ran fine afterwards. Half a tank is a bit much. If I was you I would buy a siphon and some plastic tanks to drain the lot in. What you can't get out won't be a problem. You then have some cans of diesel/ petrol mix.