This has also been added to the Quick Links in the sidebar, but a friend of mine that does various internet and computing stuff has created a website that shows all the Marginal Seats of the last election. It's sortable and filterable by parties.
One thing it does show is that UKIP really aren't the next force in politics. They only have 5 seats where they are within 5000 votes, and 2 of those are in solidly Labour areas.
Link