[It's a lullaby that his mother had sung to him as a child. He's actually surprised that he remembers all the lyrics.]

Over the hills, and o'er the main
To Flanders, Portugal and Spain.
Queen Anne commands and we'll obey
And go over the hills and far away.

[Sherlock doesn't move his arm away from her. She can use it as a pillow and he'll slip it out of her grasp as soon as her sleep's deep enough.]

There's twenty shillings on the drum
For him that with us freely comes.
'Tis volunteers shall win the day
Over the hills and far away.