This event took place on Friday, 16th April 1971 and featured Nicholas Kynaston, Marcel Dupre and Reginald Foort at the RAH organ, George Blackmore at the Hammond X66, and Jackie Brown at the Conn Theatre 3 manual organ.
Marcel Dupre was, by this time, becoming extremely frail (he died aged 85 just six weeks later) and had to be assisted up the steps to the console. He performed three pieces - Bach's Toccata & Fugue in D minor; his own Cortege and Litany and an Improvisation on two submitted themes, one of which, if my memory serves me correctly, was the English lullaby "Twinkle, Twinkle, Little Star" (I can't remember the other theme but it may have been something Patriotic such as "Rule Britannia").
I still have the printed booklet and programme for the RAH Grand Centenary Organ Festival (15p Admission!!!).