Complex sentence: The campaign was subsumed with the European Parliament elections, and the result was an all-time low turnout for a by-election: at 19.9% it holds the record for the lowest turnout in a UK parliamentary by-election in peacetime since 1918.
Split into simpler sentences: The campaign was subsumed with the European Parliament elections, and the result was an all-time low turnout for a byelection. At 19.9% the turnout holds the record for the lowest turnout in a UK parliamentary by-election in peacetime since 1918.