The IOD publish a useful factsheet on the role of the SID
Its states:
In summary, the senior independent director’s role would usually involve:
working closely with the chairman, acting as a sounding board and providing support
acting as an intermediary for other directors as and when necessary
being available to shareholders and other non-executives to address any concerns or issues they feel have not been adequately dealt with through the usual channels of communication (ie through the chairman, the chief executive or finance director)
meeting at least annually with the non-executives to review the chairman’s performance and carrying out succession planning for the chairman’s role
attending sufficient meetings with major shareholders to obtain a balanced understanding of their issues and concerns