Don't feel stupid. Motoring organisations reckon misfuelling happens every three minutes with up to 150,000 cases a year. You can usually get away with up to 10 litres of petrol in a 60-litre tank, so you should be okay. But you should fill up with high-lubricity premium diesel and keep doing exactly that. Don't allow the tank to be more than half-empty for about 2000 miles. At first, the petrol will float on top of the diesel and won't get sucked into the low-level fuel pick-up before dispersing into the diesel fuel.