The Cockatrice boys - Joan Aiken

On a rainy September Sunday in the near future a cargo of strange, shapeless bags appears on the luggage carousel at Manchester Airport. The bags are huge, and some people think they move, but everyone is quite willing to forget them when they
disappear. Only later, when Britain has been devastated by the cockatrice monsters they contain, is this recognised as the beginning of disaster. A train is sent with forces to do battle with the monsters and with tinned carrots to feed the starving population of Manchester. The last recruit to the train is a drummer boy, Dakin and his adventures are told in this imaginative story.