Okay, according to my Oxford Companion to Ships & the Sea (a mighty tome). "H. of O. was written, as we know, by David Garrick. It was set to music by William Boyce in 1759 and first performed in the pantomime Harlequin's Invasion. It commemorates the achievements of 1759 (...)during the 7 years War when the army had triumphed at Minden, the navy at Lago and Quiberon Bay and both together at the capture of Quebec. It was later traditionally played on board British Ships of the Line when they sailed into battle (see also Britons strike Home) and when the drums on board beat to quarters they did so to the rhythm of H. of O." So there we go.....

Mo