In 1981 a question was introduced about how many cars and vans were owned by each household. Topics in the 1971 Census which did not appear in the 1981 Census were: parents' countries of birth; address five years ago; occupation one year ago; dates of marriage and details of children born in marriage; certain household amenities (such as cooker, kitchen sink and hot-water supply).
You can see the actual census form that was filled out in 1981.