Not good news. Depends on the ratio. You can get way with a small amount of petrol with diesel (Before diesel additives the LT used to add petrol to diesel in small amounts to their buses in Winter. Stopped the Derv gelling and gumming up the works.
If you have put a full tank of petrol in a diesel then the AA will come out at cost and drain your tank at specified places.. I once put about 10 litres of petrol in my derv tank but I topped up with the correct diesel (about 50 litres) No problem.