For example, according to this a swing from Labour to Conservative of 20% gives Labour 15%, COnservatives 52% and the Lib Dems 22% and an overall majority for NIck Clegg’s army of 71 with 359 seats. Now, I don’t think 22% of the vote will give you 359 seats, even under First past the post, but I could be wrong.
So, note to the Torygraph bods – go back to your constituences and fix your software.
ETA They have now fixed it. A 20% swing to the Tories would give them 524 seats and the Lib Dems 27.