Assuming you've already done the obvious (blotting up as much petrol as possible and then leaving the doors open to let the car air out) - our next steps would be to try some cleaning methods using stuff you might already have around the house. The first we'd try is to put some coffee grounds on the affected areas and then leave them for a few hours. Vacuum them up and then scrub the carpets in the car with carpet shampoo. Then leave the car to air dry in the sun again and spray something like Febreze in there.

Alternatively, get some old towels that you don't mind getting dirty (and probably binning) and some hot water, baking soda and white vinegar. Once you've tried blotting up the petrol with the towels, mix some hot water with vinegar in a bowl. Apply the vinegar and water solution to the area of the carpet that you spilt petrol on. Use a wet and dry vacuum or a towel dry it. Then cover the area with baking soda and leave it for a few hours - the longer the better probably. Use the vacuum or a rag again to wipe off and clean it. Then use hot water to clean the area and allow it to dry in the sun. Again, try Febrezing it when you're done. If both methods still don't get the smell out, try laundry detergent on the carpets, just make sure you use gloves to wipe it clean.