Jeremy Brett is suffering conjunctive heart failure and manic depression in the last episodes, particularily in "The Mazarin Stone". HE FOUGHT MENTAL ILLNESS ALL HIS LIFE AND HIS MEDICATION CONFLICTED WITH HIS HEART.
He appears for a few minutes at the beginning and end. He died in his sleep in 1995. In the end interviews, much is revealed on disc 12.
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Sherlock Holmes. The Complete Granada Television Series