Ecellent Martin!
It seems the story is at least plausible.
I checked the sunrise time for Flensburg at the Naval Oceanography Portal* and it was 4.55 AM at that time of year (3.55 AM British time?) while civil twilight was at 4.14. That would make the 19./20. April mission the most probable date for the described event. In the darkness of night no-one could have regonized flags. After twilight and/or sunrise, yes. For 19./20. the problem is mission area would not take 305 sqn upto the railwayline from Odense to Berlin?
Another possible date would be 25./26. Precisely the right area and no recorded attacks, even. The problem is that at 25th the troops (and train) should already have been in Berlin. Possibly there was several echelons transported during a period of time?
Anyway, thank you a million for this piece of information!
Regards,
Kari
*
Complete Sun and Moon Data for One Day: Locations Worldwide( http://www.usno.navy.mil/USNO/astronomical-applications/data-services/rs-one-day-world )