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, wherein in the secondary cell group one cell is designated as a primary secondary cell, and the master cell group and the master base station are configured within a Long Term Evolution radio access network, and the secondary base station is a core unit associated with distributed units providing the cells of the secondary cell group, the core unit and the distributed units configured under a different radio access network, the master cell group and the secondary cell group interworking to provide the wireless telecommunications network; and a terminal device configured to communicate wirelessly with the base stations including by the use of a split radio bearer receivable at the secondary cell group via the primary secondary cell for splitting between the secondary base station and the master base station before delivery to the terminal device; the method being characterized by further comprising: in the event of a need to change the designation of the primary secondary cell from a first cell to a second cell in the secondary cell group when the first cell and the second cell are both provided by a same distributed unit so that the first cell and the second cell share a common medium access control protocol layer, the change of the primary secondary cell from the first cell to the second cell is performed using signalling in the common medium access control protocol 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, wherein in the secondary cell group one cell is designated as a primary secondary cell, and the master cell group and the master base station are configured within a Long Term Evolution radio access network, and the secondary base station is a core unit associated with distributed units providing the cells of the secondary cell group, the core unit and the distributed units configured under a different radio access network, the master cell group and the secondary cell group interworking to provide the wireless telecommunications network; and a terminal device configured to communicate wirelessly with the base stations including by the use of a split radio bearer receivable at the secondary cell group via the primary secondary cell for splitting between the secondary base station and the master base station before delivery to the terminal device; the method being characterized by further comprising: in the event of a need to change the designation of the primary secondary cell from a first cell to a second cell in the secondary cell group when the first cell and the second cell are both provided by a same distributed unit so that the first cell and the second cell share a common medium access control protocol layer, the change of the primary secondary cell from the first cell to the second cell is performed using signalling in the common medium access control protocol layer.