you can drive with a mix of petrol / derv and it will be ok. not to much petrol though.
it use to be, diesel would freeze in the winter. to get around this you could say put 5 galls of derv in, then top up with 1 gall of petrol.
I've done that many times in taxi ( black cab ).
Did the same as you once. cab cut out a mile later. We saved the mixed fuel and each time we filled up after, added a can of it to the tank.