How much of the decline in participation is due to people knowing the outcome was a foregone conclusion? It's hard to infer the Labour support when people were confident it would be a wipeout so they may not have felt a pressing need to go to the polls.
when overturning a big majority like this, you'd put your limited resources deeper into Tory territory too, and to promote unfamiliar candidates. Meaning you'd spend less time getting the vote out in your safer seats. So you'd think there were easy wins in terms of vote count.
