The Trust has released a statement saying Maria Ball has resigned with immediate effect.
The process for appointing a new or interim chair will be started immediately but in the meantime Paul Remington who is vice chair will lead the organisation with Andrew Morgan, the interim chief executive.
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Saturday will be largely cloudy with a continued risk of heavy showers or thunderstorms. Sunday will be dry with sunny spells.
There continues to be a risk of thunder tonight and tomorrow but the weather will become drier and clearer by sunday.
The Chief Exec of car manufacturer Lotus has said he is deeply sorry to announce job losses at the company's head quarters in Norfolk.