A method for use in a wireless telecommunications network, the method including: when a 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 a 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 a terminal device; and the master base station, in response to the notification, operating to effect an alteration in the said data handling resources.