The deal, for an undisclosed sum, swells the group's UK branch network to 83.
Mike Hutton, Equity's acquisition director said: "We are well on target to achieving an anticipated 100 branches before the end of 2008."
Established in 1984, Castle Insurance Services was owned by Paul Matthews, who acquired it in 2004 when founder, Jack Neill retired.
Mr Hutton added: "The Castle acquisition provides a strategic extension to our existing East Anglian branches in King's Lynn and Swaffham."
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