Formed in 1973, S.O.S. brought together three British sax giants, Surman, Osborne and Skidmore, for a project that turned out to be much more than merely a saxophone trio. Sure, their unaccompanied three-part horn constructions are entirely gripping and imaginative – with the hugely under-appreciated Osborne on particularly penetrating form. But it was Surman and Skidmore's multi-instrumentalism that broadened the palette. The live recordings from 1974 and 1975 included on this collection of rarities really bring that to the fore. Skidmore's drumming is loose and free, while Surman throws in burbling analogue synth loops and chunky organ jams as swirling segues, enabling the trio to meld compositions into the kind of continuously flowing live set that Soft Machine favoured. A lengthy live track recorded at London's Queen Elizabeth Hall, with an athletic turn by Tony Levin on drums, brings even more density and bite to the group sound. S.O.S. only managed to cut one album, in 1975, before disbanding the following year, so this double-disc effectively triples their recorded output: great news for fans of 1970s Brit-jazz and scholars of the prog-jazz continuum.