Rooftoppers by Katherine Rundell

Everyone tells Sophie that she was orphaned in a shipwreck – found floating in a cello case on the English Channel on her first birthday. But Sophie is convinced her mother also survived. When the Welfare Agency threatens to separate her from her guardian and send her to an orphanage, Sophie takes matters into her own hands, starting with the only clue she has – the address of a cello-maker in Paris. On the run from the authorities, Sophie finds Matteo and his network of rooftoppers – urchins who walk tightropes and live in the sky. In a race across the rooftops of Paris, will they be able to find her mother before it's too late?