Bonnie Tyler built one of pop's most peculiar careers: two bursts of global success that, beyond the name on the record sleeves, seemed to have almost nothing to do with each other.

Her first major British hits arrived in quick succession. "Lost in France" charted in 1976, followed by "It's a Heartache" in 1977 — both superior examples of what writer Pete Paphides subsequently dubbed "medium wave pop": the largely forgotten material that actually filled the charts and Radio One's playlists at a time when reductive rock histories would have you believe the entire nation was gripped by punk.

The early hits sat at the intersection of soft rock and country, reminiscent of reliable mid-70s hitmakers Smokie. They were so catchy that no one seemed to notice something had changed between the two releases. Tyler's voice possessed a rather sweet tone on "Lost in France"; by the time of "It's a Heartache," an operation to remove nodules on her vocal cords had given her a striking Rod Stewart-like huskiness.

That operation inadvertently created her signature sound. "It's a Heartache" sold 6 million copies, and the accompanying album reached the Top 3 on the US country chart. It looked as though the single would turn Tyler into a huge star.

Sustaining that success proved difficult. Her record label seemed bizarrely unsure what to do with her. Should she cover Tom Petty and the Heartbreakers, as on "Louisiana Rain"? Should the label aim her squarely at the easy listening market via a version of "Sometimes When We Touch"? Or push her toward disco, as on the fabulously camp "(The World is Full of) Married Men"?

The label tried all three. None stuck.

Tyler's second act came when she found her groove with Jim Steinman-penned epics, shining through even the most overblown backing tracks. After hopping between country, disco and soft rock, she eclipsed her power-ballad peers and built an astonishingly wide variety of pop — a range few of her contemporaries could match.

The trajectory from Swansea clubs to worldwide fame, traced in a companion photographic retrospective, charts a singer who refused to be pinned to a single genre. Tyler's career now stands as a case study in how a distinctive voice — one altered by surgery rather than design — can outlast every marketing strategy imposed upon it.