We have a frustratingly cloudy picture of JS Bach in his final years (1735-1750), but this week Donald Macleod looks at the composer's preoccupations during this period, when it seems that he was contemplating past, present and future. One biographer suggests that towards the end of his life, as he sat at his composing desk at St Thomas's School in Leipzig, he would have been surrounded by the 'Old Bach Archive' - the music of his ancestors - on bookshelves. He had recently drawn up a family tree, and was proudly watching his sons begin to make their mark in the musical world. From the late 1730s onwards Bach began to retreat from his church duties in Leipzig, possibly in protest against his employers, and started to devote himself to his own large and ambitious projects. One of these was the Mass in B Minor, a monumental work which seems to have been written without a commission or any intended performance, and which will be heard in its entirety through the course of this week's programmes.