I wonder if you might be able to smell any residual petrol in the soil? Try taking a few handfuls and give it a sniff?
If it was diesel fuel, you'd definitely have to dig out some soil and replace it. Oil hangs around a long time and messed up the plants badly. Gasoline should evaporate eventually, I would think. Perhaps a douse with some mild, soapy water will rinse out the residue, and won't hurt the plants.
A call to the fire brigade might also give you a hint, as to whether they used some sort of fire suppressant rather than just water.
