Paul Shaviv has a fascinating post on Sir Isaiah Berlin and Slifkin-gate.
It seems that Sir Isaiah, Yehudi Menuhin and the 7th Lubavitcher rebbe were all equal Schneerson cousins and the rebbe's wife, Chaya Mushka (and daughter of the 6th rebbe, Yosef Yitzchak Schneersohn) used to travel to London to see Sir Isaiah's mother and attend the theater with her.
By the way, Sir Isaiah – an infrequent synagogue attendee – inherited from his mother the tefillin of the Alter Rebbe, Shneur Zalman of Liady the founder of the Chabad dynasty. It was not uncommon for Lubavitch hasidim to make a pilgrimage of sorts to Sit Isaiah's home just to catch a glimpse of them.
Does anyone know where they are kept today?