Complex sentence: His loyalties are therefore split between his Minister and his Civil Service boss, Sir Humphrey: while he is theoretically responsible to Hacker personally, it is Sir Humphrey who writes his performance reviews and influences Bernard's Civil Service career.
Split into simpler sentences: His loyalties are often split between his Minister and his Civil Service boss, Sir Humphrey. While he is theoretically responsible to Hacker personally, it is Sir Humphrey who writes his performance reviews and influences Bernard's Civil Service career.