One of the outcomes of a first-past-the-post electoral system is that it creates safe seats. These are seats where one party has such a large lead over their opponents in their constituency that the chances of being overturned is slim.
It seems obvious that people in these seats will be less minded to go out and use their vote. The statistics back this up.
This graph compares the size of the majority (the scale of the win) in the 2001 general election to the size of the turnout in 2005 in each constituency.
(Excludes Northern Ireland)
There is a significant trend (PMCC -0.71). As the majority at the last election gets bigger, the turnout tends to goes down.
If we are to get people voting again, there needs to be an end to the first-past-the-post electoral system that gives MPs unassailable majorities.
UPDATE 26/01/2010: Timothy asked if the scatter plot could be colour-coded by party. Happy to oblige. Each dot is colour-coded by the winning party in 2001.
So it does seem that the seats with the big Labour majorities 2001 subsequently had low turnouts in 2005. I'm not sure this shows much, since the Tories had no heavily safe seat in 2001. I'll definitely remake this plot after the next general election to see if the same pattern occurs.