 At the 1966 election, she lost her seat to Labour's Alistair Macdonald, by a majority of only 810.Four years later, at the 1970 election, she regained the seat with a majority of 3363.Constituency boundary changes implemented in the February 1974 general election encouraged Hornsby-Smith to allow Roger Sims to stand for Chislehurst, and to compete instead for the new constituency of Sidcup.