Go here. (It’s a campaign put together by quite an impressive coalition of groups including the Fawcett Society, Greenpeace, Friends of the Earth and Avaaz.)
Why is it needed? Consider these figures:
Conservatives: 35,021
Labour: 33,338
Liberal Democrat: 119,397
That’s the number of votes each party needed to win a seat.
(Figures from here.)
Feminist Philosophers 
Would one of the UK-based posters be willing to explain where those numbers come from? I’ve seen them around, but not with an explanation that I can understand.
My understanding is that the Lib Dems are geographically scattered amongst constituencies in such a way that their numbers don’t translate well to seats. (I don’t know how much of that is due to deliberate action on the part of the Tories and Labour.)
It *might* be partly to do with the fact that Lib Dem voters sometimes tactically vote Labour to keep the Tories out. But Tory and Labour voters *never* team up to stop the Lib Dems from getting in.
Definitely true Monkey. But how does that explain the way that Lib Dem votes don’t translate into seats?
It could also be that more of the seats the Lib Dems won were in highly populous districts. Looking at the popular vote, the Lib Dems are much closer to Labor than they are on seats:
http://en.wikipedia.org/wiki/United_Kingdom_general_election,_2010
My last comment wasn’t intended to explain why Lib Dem votes didn’t translate into seats. It was just commenting on the suggestion that it might have been due to deliberate action by Labour and Tory.
As you say, it seems that the reason Lib Dem votes didn’t translate into seats is because they’ve been scattered across constituencies, rather than forming concentrated pockets within constituencies that enables them to win seats.
Also, the size of the constituencies varies. I’ve seen some figures that show the smallest constituencies are Labour strongholds, so those seats effectively ‘come cheap’ as Labour don’t need so many votes to win a seat. Some of the largest constituencies are Tory strongholds, so those seats are more expensive, needing more votes to win. If Lib Dem voters are concentrated in the larger constituencies, there can be lots of votes, without them gaining a seat.
Consider that UKIP and BNP together got 5%. In this case Labor and LD had over 50%, but if they got into power that way it’d only be a matter of time until the Tories and extremists get 50% (it’s the nature of politics to swing from side to side as people realize their government is lame, don’t imagine you’d get perpetual Labor-LD rule… part of why the LDs did well is because they haven’t had to govern and so can’t be blamed) and then the Tories would have to give the extremists veto power over everything they do (don’t imagine they would instead jump to a unity government either).
Such a system would definitely be more fair, but it might not lead to better government.
Anonymous: I agree that one outcome of a more representative system is that these small and sometimes extremist parties may be represented in parliament.
However, I think (even) the tories would be wary of associating with the BNP – it could be very damaging, politically. (Though I’m not sure the same can be said about UKIP.) In a situation in which they did not alone have a majority, I should imagine that they’d rather align with independents or other small parties (of which there might also be more, under an alternative system).
It’s true. But even if they do associate with other small parties, these would then have power far beyond the percentage of votes they got because without them the coalition would collapse. Whether that’s good or bad depends on the parties involved and their specific demands. Either way, though, it doesn’t lead to power in influencing policy actually being proportional to the fraction of votes. The alternative is some kind of big-parties coalition, but they usually don’t like those because each one wants to have the prime ministry.
Anyway, I think of it as a crapshoot: it might be better, but it’s impossible to predict or gauge the odds. Obviously the current system is very problematic for its own reasons.
I seem to recall that Shas, an Israeli Ultra-Orthodox party, would up with a disproportionate amount of power for a while by doing exactly what’s described in 7 and 9.