In February 1974, neither Labour nor the Tories got a majority; after a few days of the Tories trying and failing to persuade the Liberals (forerunners of the Lib Dems) to join them, Labour became a minority government, and after a few months of increased popularity, they called another election, they got a very small overall majority. After a few by-elections, that disappeared too, and they had a 'Lib-Lab' pact in which the Liberals generally supported Labour for some time. Eventually that fell apart, and Labour lost a vote of confidence (only a few months before they'd have had to call an election anyway) in 1979. And right at the end of John Major's term up to 1997, the Tories were a theoretical minority government, though they could usually get votes from like-kinded right-wingers from Northern Ireland to win any vote.
If people want to read more, I reccomend this blog:
http://hungparliament2010.blogspot.com/ which looks into all the possibilities. You can also read fivethirtyeight's take on it:
http://www.fivethirtyeight.com/2010/01/hung-parliament-from-gallows-perhaps.htmlNotice there are a few other parties to take into account - the various Northern Ireland parties (note that Sinn Fein never take their seats in parliament, because it involves an oath or affirmation of loyalty to the Queen and her successors, which they won't take; so the seats they win should be subtracted from the total votes that can be cast in a parliamentary vote), the Scottish and Welsh nationalist parties, one or two independents with local popularity for various reasons, and one seat that the Green Party has a genuine chance of winning. So it won't necessarily be the Lib Dems involved in coalitions (though since Labour and the Scotttish/Welsh nationalists are the biggest parties in those nations, it's hard to see them working formally together at Westminster).