Agaricus Bisporus,
I've done a bit more delving and you are right to express doubt. According to the ab-ix website, it looks as though the Genet Moths were not delivered to CFS until June 1927 - a tad late for the 1926 King's Cup!. There is ANOTHER Genet listed, though - G-EBOU which was registered to the De Havilland company in July 1926, CofA'd in Sep 1926 and flown by Hubert Broad in the Lympne Trials. Looks like only the King's Cup archives will resolve this!