Complex sentence: The Conservative Party and Liberal Democrats have both been very weak in the seat.From 1987 until 2017 neither had ever won 10% of the vote and the Conservatives had never achieved one eighth of the total votes cast.
Split into simpler sentences: The Conservative Party and Liberal Democrats have both been very weak in the seat. Neither has won 10% of the vote since 1987 and the Conservatives have never achieved one eighth of the total votes cast.