A radio network control node acts across an Iur interface as a drift radio network control node (262) for a user equipment unit (UE) in a communications system supporting a multimedia broadcast multicast service (MBMS). The drift radio network control node (262) maintains a first counter for counting a number of events occurring at the drift radio network control node (262). Registration of the drift radio network control node (262) with a core network node (30) is advantageously delayed until the counter has exceeded a first threshold value. In one example mode, the number of events occurring at the drift network control node (262) which is counted by the counter is a number of user equipment units for which a Iur linking procedure is performed for the MBMS session. In another example mode, the number of events occurring at the drift network control node (262) which is counted by the counter are time periods elapsed since an Iur linking procedure for the MBMS session has been performed for a predetermined user equipment unit.