I've been thinking about general election swings. If you look at the Butler Swing of the big landslides of the 20th Century then the biggest ones are:

1906 Liberal 6%
1945 Labour 12%
1979 Conservative 5%
1997 Labour 10%

Because of the large number of Liberal Democrat votes at the last election, it is difficult to calculate the national swing needed to overturn Labour's majority. However, a swing of over 8% would appear to be needed . This would be greater than they had achieved even in 1979. The large landslides of the century also have tended to be to the left rather than the right, even further showing the mountain the needs to be climbed.