Very easy to explain the basis for this; all the music is composed on a base ground and it is all by Henry Purcell - originally. However, it has all undergone transformation. In one case, the Round, to a stripped down version by Purcell himself (much better known being the orchestrated rondeau from his suite for the play Abdelazer, although it was also appropriated by Benjamin Britten). In the others the re-imagining is much more recent and allows a modern sensibility full play - resulting in a fusing of styles in the tradition of Purcell himself, and in the case of Nyman something approaching a complete reimagining. Maybe it helps if you also love the film of the Draughtsman's Contract (as I have to admit, having first seen it as an undergraduate, to still being smitten after more years than I would necessarily care to remember). So don that frock coat and periwig - and reach for the saxophone.