String Quartet No. 12 was commissioned for the 1988 Nottingham Festival and completed on October 22, 1987. The Coull Quartet, which has a close connection with Simpson's music, having previously performed the first concerts of Quartets Nos. 10 and 11, gave the premiere performance. The work is dedicated to Lionel Pike, a composer and musicologist.[5]
The composition is divided into two extended movements. The first movement is characterized by its slow, contemplative mood and pronounced polyphony, followed by a monumental Scherzo. Structurally, this Scherzo bears similarities to the finale of Simpson's Symphony No. 11 from 1990. Beginning with a tranquil Adagio, which introduces the first violin with a peaceful fugue theme, the music gradually increases in urgency and finally ends quietly.
The second movement, a Molto vivace, features an energetic Scherzo, characterized by small intervals such as thirds and fourths. It adopts Beethoven's characteristic simple triple meter and combines powerful and humorous elements before abruptly concluding with a dynamic finale.
The String Quintet was commissioned by the BBC for the Monday midday concert series at St. John's, Smith Square, London. It predates the Twelfth Quartet and, like Mozart's string quintets, employs two violas. The work consists of a single, large-scale movement with two main tempi—Andante and Vivace—interwoven in a style typical of Simpson's compositional approach.
The quintet begins with a simple melody played by the first violin, which is then taken up by the other strings, leading to a majestic symphonic conclusion. This work is considered one of the most important of its kind since Brahms's Opus 111.