sentences:  At the 1979 general election, Douglas-Mann had held the seat with a majority of only 618 votes over the Conservatives.Douglas-Mann decided in early 1982 to join the recently formed Social Democratic Party (SDP); several other MPs had already joined the party.Although there was no requirement to do so, he decided to resign his seat and stand in the resulting by-election in order to gain a mandate for his change of allegiance.