I'm sure we have had this many times before, I am pretty sure diesel floats on petrol though... sloshing it around in the tank will mix it up but it will percolate back together again when the car is left standing, but I may be wrong. There is a lubricant in diesel which is not present in petrol, this is for the fuel pumps and injectors. Get the petrol out ASAP and cross your fingers, hope you get away with it.