George and the Unbreakable Code by Lucy Hawking

George and his best friend Annie haven't had any space adventures for a while and they're missing the excitement. But not for long . . . 

Seriously strange things start happening. Banks are handing out free money; supermarkets can't charge for their produce so people are getting free food; and aircraft are refusing to fly. It looks like the world's biggest and best computers have all been hacked. 

George and Annie will travel further into space than ever before in order to find out who is behind it.