1. A data transfer system comprising: a server; and a plurality of devices each operable as a client of the server and as a peer-to-peer device; the server comprising: a notification arrangement to notify the plurality of devices that they are to form a peer group; and a client/server data transmitter to send data to the respective devices upon such notification; each device comprising: a client/server data receiver to receive respective data from the server; and a peer-to-peer communicator to send the respective data received from the server to one or more peers, and to receive from one or more peers the respective data that was sent to it or them by the server.