Louise Mensch's resignation means that there will now be three by-elections on 15 November: Corby, Cardiff South and Manchester Central. Of note is that Labour stands to win all three. It currently has a majority of 4,709 in Cardiff and a majority of 10,430 in Manchester. It lost Corby to the Tories in 2010 after holding the seat for 13 years but should have little trouble overturning the Tories' majority of 1,951.
A triple by-election defeat is not a happy prospect for David Cameron as he seeks to relaunch his administration. Conversely, it will strengthen Ed Miliband's reputation as a winner (Labour has won five by-elections since he became leader), leaving Bradford West as the only blot on his record.
Here's the full result from Corby in 2010.
Louise Bagshawe (Conservative) 22,886 (42.2%) +2.3%
Phil Hope (Labour) 20,935 (38.6%) -4.5%
Portia Wilson (Liberal Democrats) 7,834 (14.5%) +1.7%
Roy Davies (British National Party) 2,525 (4.7%) +4.7%
Majority: 1,951 (3.6%)