Complex sentence: The Mk 5 twin 40 mm CIWS batteries were heavy and required too many ratings to maintain and pre Falkland war experience not assessed as useful against the expected scenario of missile, standoff and high level attack.
Split into simpler sentences: The Mk 5 twin 40 mm CIWS batteries were heavy and required too many ratings to maintain. Pre Falkland war experience, they were irrelevant to the expected scenarios of missile, standoff and high level attack.