General Election 2015: What is the leaders' debate worm?

The leaders of seven political parties are gearing up for a live, two-hour televised general election debate.

The contest, announced after weeks of wrangling, will be the only time Conservative PM David Cameron and Labour's Ed Miliband will face one another in a debate before polling day.

The Lib Dems, SNP, UKIP, Plaid Cymru and the Greens will also take part.

As the leaders debate, the BBC will gauge their effect on an audience of 50 undecided voters.

Reeta Chakrabarti explains.