The LibDems said at some point that one should only hold a referendum on a topic that one is in favour of. And so they supported a referendum on the European Constitution (which they're in favour of), but oppose one on Scottish independence (which they are against). A bit strange that they abstained on the question of a referendum on the Lisbon Treaty, though, given that they were in favour of that, too. Makes you wonder whether the principle is fully set in stone.
Labour seems to be of the opposite opinion: They were against a referendum on the Lisbon Treaty (which they supoorted), but they now seem to be in favour of a refendum on Scottish independence (which they're against).
I actually tend to agree with Labour here: A party is voted into power to implement their policies. It therefore doesn't need a referendum to get a mandate to do so. However, if there is something the party doesn't want to do, but that is very popular in the wider population, it can make sense to hold a referendum to defuse the issue. It's a bit masochistic, though, and most parties would prefer to delay the issue instead.
I'm not very fond of referendums anyway. They tend to be about everything else, and they often get quite emotional. And in many cases, the major parties can only really live with one result, which tends to provoke the electorate to vote for the other option in spite.
I tend only to support referendums on topics that transcend political parties, such as Scottish independence. I am looking forward to that one, just like Bendy Wendy! 😉