Yep as above, just flush out what you can and it will be fine. A bit of petrol left in the bottom of the tank with a whole tank of diesel will do nothing at all given the dilution factor, probably clean out the injector nozzles nicely! You already have a 50/50 mix, so the dilution is going to be even greater than if it was a whole tank of petrol ( as usually happens ).
For the record my brother topped his mechanic apprenticeship back in the day, and is quite the mechanical guru, and he's done this, so don't feel so bad.
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