syphon out the contaminated fuel, either by getting a tube into the fuel filler if possible and sucking it out, or disconnect the fuel line under the bonnet, turn on the ignition and allow the electric pump to pump it through and into a bucket, Im sure I dont need to tell you to be very carefull, no smoking or sparks!!
Then put plenty of fresh petrol in the tank, this will help dilute any left over diesel. Reconnect the fuel line and start it up, It may take a while to start and once its running it will probably cough and splutter for awhile but should clear after a few miles.