Message in a bottle

Meet Sailor, Moses, Wader and Shrimp. A gang of four Victorian children condemned to searching through the sludge under Putney Bridge for anything they can sell to make ends meet - the Mudlarkers! Wader has found a letter in a bottle, washed up on the riverbank. The note is mysterious and confusing and it's followed by another one the very next day. Who is sending them? Why is Wader the only one who can read them? And what is this magic that he's being warned of, about to befall the river?