A communications system is described in which a base station controls interworking between a cell of the base station operating according to a first radio access technology and cells of access points operating according to a second radio access technology. The base station transmits control plane data to a mobile device in its cell, wherein the control plane data facilitates user plane communication by the mobile device, via a first access point. When the user plane communication has been handed over from the first access point to a second access point, the base station receives information identifying the second access point. Beneficially, the base station uses the received information for controlling the interworking and/or controlling a handover for the mobile device.