It is from the special episode called Music Of The Spheres which David filmed to be shown at the first Doctor Who Proms in the Royal Albert Hall in 2008.
EmpireOnline has an exclusive 360° panoramic tour around Sherlock's 221B Baker Street Set which includes the designer Arwel Wyn Jones talking about the individual props in the BBC's set of 221B Baker Street.
To find the prop and narrative, rotate the photo (using the arrow below it) to the right and click on the sheet music on the music stand.
Link to EmpireOnline article and graphic.
Direct link to MP3 of Arwel Wyn Jones talking about the sheet music.