Thanks also Ross. You beat me to going back to the excellent 211 archive. I just did, and there is the answer:

Geary AC Pilot Officer 79188 Distinguished Flying Cross Air Ministry AIR 2/9532
“This officer has completed 62 sorties of which 38 were carried out within 42 days. He has acted as rear gunner in his Commanding Office’s aircraft for six months, and his keen observation and clear reports on the numerous manoeuvres of enemy aircraft have undoubtedly helped to save our own aircraft on many occasions. He has assisted in beating off enemy fighters on 16 occasions. His courage and devotion to duty have been exemplary.”

London Gazette 13 March 1942 p1164 “Awarded with effect from 21 March 1941…
For gallantry and devotion to duty in the execution of air operations”

So, for long and faithful service rather than a single act of bravery. I must say it’s moving to have a DFC in the family. It’s the first to my knowledge!