A method for use in a wireless telecommunications network, the mobile telecommunications network comprising: a core network; base stations supported by the core network and each providing wireless connectivity within at least one base station cell where the cells are arranged into a master cell group under control of a master base station and a secondary cell group under control of a secondary base station; and a terminal device configured to communicate wirelessly with the base stations by the use of a split radio bearer receivable at the secondary base station for splitting between the secondary base station and the master base station before delivery to the terminal device; the method comprising: when the secondary base station exhausts a supply of unique parameter sets used in security ciphering of received split radio bearers, the secondary base station sends a notification to the master base station of a requirement to alter data handling resources allocated for handling split radio bearers received from the secondary base station in one or both of the master base station and the terminal device; and the master base station, in response to the notification, operates to effect an alteration in the said data handling resources by re-establishing its radio link control protocol layer and not resetting its medium access control protocol layer, and sending a reconfiguration message to the terminal device to effect re-establishment of its radio link protocol layer but not resetting of its medium access control layer. a core network; base stations supported by the core network and each providing wireless connectivity within at least one base station cell where the cells are arranged into a master cell group under control of a master base station and a secondary cell group under control of a secondary base station; and a terminal device configured to communicate wirelessly with the base stations by the use of a split radio bearer receivable at the secondary base station for splitting between the secondary base station and the master base station before delivery to the terminal device; the method comprising: when the secondary base station exhausts a supply of unique parameter sets used in security ciphering of received split radio bearers, the secondary base station sends a notification to the master base station of a requirement to alter data handling resources allocated for handling split radio bearers received from the secondary base station in one or both of the master base station and the terminal device; and the master base station, in response to the notification, operates to effect an alteration in the said data handling resources by re-establishing its radio link control protocol layer and not resetting its medium access control protocol layer, and sending a reconfiguration message to the terminal device to effect re-establishment of its radio link protocol layer but not resetting of its medium access control layer. when the secondary base station exhausts a supply of unique parameter sets used in security ciphering of received split radio bearers, the secondary base station sends a notification to the master base station of a requirement to alter data handling resources allocated for handling split radio bearers received from the secondary base station in one or both of the master base station and the terminal device; and the master base station, in response to the notification, operates to effect an alteration in the said data handling resources by re-establishing its radio link control protocol layer and not resetting its medium access control protocol layer, and sending a reconfiguration message to the terminal device to effect re-establishment of its radio link protocol layer but not resetting of its medium access control layer.