A mobile data communications system comprising: a mobile device having a reconfigurable user identification module operable to store a primary mobile identity and a further mobile identity, the mobile device being operable so as to provide mobile data communication via a mobile network selected from a set of available mobile networks in accordance with the primary mobile identity, subject to the primary mobile identity being registered with the selected mobile network; and an account control server comprising: a memory to store identification and payment details for a user of the mobile device and a mobile identity provider to provide a mobile identity to the mobile device and to an authorisation server (60, 70)of a mobile network; the mobile device and the account control server being operable to communicate via a mobile network in accordance with the further mobile identity; the account control server being arranged so that, in response to a request by a user of the mobile device to obtain data access by a particular mobile network, the account control server provides a mobile identity to the mobile device for storage by the user identification module as the primary mobile identity, provides the same mobile identity to the authorisation server of that particular mobile network, and provides the identification and payment details relating to that user to that particular mobile network; the reconfigurable user identification module being operable to use the further mobile identity at least for communication with the account control server in respect of subsequent changes to the primary mobile identity; and the reconfigurable user identification module being operable to overwrite a current primary mobile identity with a mobile identity newly received from the account control server. a mobile device having a reconfigurable user identification module operable to store a primary mobile identity and a further mobile identity, the mobile device being operable so as to provide mobile data communication via a mobile network selected from a set of available mobile networks in accordance with the primary mobile identity, subject to the primary mobile identity being registered with the selected mobile network; and an account control server comprising: a memory to store identification and payment details for a user of the mobile device and a mobile identity provider to provide a mobile identity to the mobile device and to an authorisation server (60, 70)of a mobile network; the mobile device and the account control server being operable to communicate via a mobile network in accordance with the further mobile identity; the account control server being arranged so that, in response to a request by a user of the mobile device to obtain data access by a particular mobile network, the account control server provides a mobile identity to the mobile device for storage by the user identification module as the primary mobile identity, provides the same mobile identity to the authorisation server of that particular mobile network, and provides the identification and payment details relating to that user to that particular mobile network; the reconfigurable user identification module being operable to use the further mobile identity at least for communication with the account control server in respect of subsequent changes to the primary mobile identity; and the reconfigurable user identification module being operable to overwrite a current primary mobile identity with a mobile identity newly received from the account control server.