One of the glories of U2 is that they have so often been out of step with the pop zeitgeist – which goes a long way to explain their longevity while so many around them have fallen victim to the shifting sands of fad and fashion. They arrived in the early ’80s into a world of synthpop as rock’n’roll crusaders with a heart-warmingly old-fashioned belief in the revolutionary potential of music and a passion and idealism that was magnificently out of step with the post-punk cynicism and insularity of the time.

From their debut album Boy (1980), they sounded huge, Bono’s passionate wail and the Edge’s ringing, skyscraping guitar brimming with optimism and self-belief. On albums such as War they turned sonic bombast into high art and by the mid-’80s they were well on their way to becoming the biggest and most important band in the world. Their crown was secured by a charismatic Live Aid performance and their position rendered unassailable by 1987’s Joshua Tree, which for many remains their masterpiece.

Of course, they left themselves wide open to accusations of self-importance, zealotry and egoism, particularly once Bono added statesman to his other job titles of showman and poet. But the second glorious thing about U2 is that they are so convinced of their own righteousness and integrity that they simply don’t care.

Into the ’90s they blithely ignored both grunge and Britpop and reinvented themselves with the post-modern dance-rock of Achtung Baby and a not-entirely successful fringe flirtation with electronica on 1997’s Pop. By the start of the new millennium they even had Salman Rushdie writing lyrics for them and they recently returned to reclaim their ‘biggest band in the universe’ tag with the widescreen melodrama of 2004’s How To Dismantle An Atomic Bomb.

U2 have released their new album Songs Of Innocence and are giving it away for free. The band announced the release at Apple's iPhone 6…