Complex sentence: Thain joined the Keef Hartley Band and, in 1971, they toured with Uriah Heep; they asked him to join them (replacing Mark Clarke) in February 1972, and he stayed in the band until February 1975.
Split into simpler sentences: Thain joined the Keef Hartley Band and, in 1971, they toured with Uriah Heep. Subsequently, Uriah Heep asked Thain to join them (replacing Mark Clarke) in February 1972, and he stayed in the band until February 1975.