Overview: Musical borrowings, reworkings and reinventions run through this season’s Proms. The invisible thread linking tonight’s concert really begins with Bach. A lilting chaconne from his Cantata No. 150 underpins the finale of Brahms’s Symphony No. 4, and the latter’s elegant synthesis of heart and head is itself the inspiration for American composer Elizabeth Ogonek’s Cloudline, a lyrical homage to ancient musical forms and techniques. The chaconne’s repeating patterns are echoed elsewhere in the circling bass line of Purcell’s powerful Lament from Dido and Aeneas. Cellist Guy Johnston is the soloist in anniversary-composer Saint-Saëns’s Cello Concerto No .1.