Rooftoppers by Katherine Rundell
On the morning of her 1st birthday a baby was found floating in a cello case, right in the middle of the English Channel! An eccentric bachelor just happened by on a morning walk when he spied the cello case floating along. Having never had a child he brings her home, where he uses toast as a bookmark and encourages Sophie to write on the walls. Tis a splendid arrangement until Child Services gets involved.
They are dreadful people after all and wish to remove Sophie from Charles' home and influence. This leads Charles and Sophie on a chase to Paris, where they hope to find out if Sophie's mother is alive. As it is she is the only person who can stop Sophie from going to the orphanage.