Complex sentence: Chadwick insisted on civility, and that, coupled with the retirement of some of the fellows, ensured an improvement in the atmosphere within the college, which continued after his second retirement (again to Oxford) in 1993.
Split into simpler sentences: Chadwick insisted on civility, which coupled with the retirement of some of the fellows, ensured an improvement in the atmosphere within the college. This continued after his second retirement (again to Oxford) in 1993.