There are very few scores anywhere in the world of more value than Bach's famous Mass. So fragile is it that the Berlin library where it's kept (the Staatsbibliothek zu Berlin) allows only a very few people ever to see it, let alone touch it. Choral conductor Simon Halsey and the Bach soprano Deborah York join Frances at the Library to get closer to the great German composer's extraordinary industry and to catch a glimpse of his humanity. It is often half-jokingly said that, to his fans, Bach is not so much a composer as a religion; but here, in his neat hand, are the crossings out and re-workings of a man still seeking to perfect music, much of which was written earlier in his life. Simon Halsey has described the B-Minor Mass as 'Bach's greatest hits', since in many ways it is a compilation of pieces he had composed over a number of years. The Berlin score isn't simply a fair copy of this assembly, but shows Bach still hard at work, changing his mind, rewriting - a phrase shifted here, a key modulated there - introducing new instrumentation and striving for something better. There is also an incredible technological story to tell. Bach's pages are literally thick with music - so thick that in many places the ink has actually burned through the paper, leaving it almost impossible to read. So the Library has had to split the single pages open and insert a protective sheet to stabilise the ink-burn.