His father, Antoni Deniz, was African, born in Cape Verde; he was a seaman, and married in Cardiff Gertrude Blanch, of English and African-American descent.
They moved to London, and found work as musicians in Soho; in 1937 they played for a time in the orchestra of Ken "Snakehips" Johnson.
[1][2][3] Val Wilmer wrote in an obituary: "... in Soho they encountered other people of colour forced to live by their wits.... the Denizes wanted none of it, and, through musicianship and character, distanced themselves from limiting, pre-ordained roles.
"[2] Deniz joined the Merchant Navy in May 1940, in between voyages playing music with contemporaries including Eric Winstone and Edmundo Ros, and forming his own band, the Spirits of Rhythm.
[1][3] Stanley Black, leader of the BBC Dance Orchestra from 1944, was beneficial to his career by employing him regularly and introducing him to others in the music business.