Manser was acting chairman for five months earlier this year, when Mackay took medical leave of absence to focus on his treatment following surgery for a brain tumour in April 2013.
Mackay recovered sufficiently to return to his role as non-executive chairman in September 2013, but following a recent sudden worsening of his condition, the board has agreed to his request for a further leave of absence.
The board will continue to keep the position under review.
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