How much petrol was it? As you drove 20kms there must have been a substantial part of Diesel in the tank left. I probably would fill up with pure Diesel and turn the key so that the fuel pump will work to flush the lines. Do this several times. Then crank the engine for some time. Most of the petrol should be washed out by then. Start engine as usual.
About possible damage... well, honestly, I don't know. The fuel pump can be quite sensitive but I don't think it will have suffered too much. In colder regions of this world Diesel cars are commonly run on a Diesel/petrol (90/10) mixture without any detrimental effects.
Good luck.
Johannes
There are people who spend all weekend cleaning the car.
And there are people who drive Discovery.
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