the leaders of britain 's seven main political parties went parked .
nobody is expected to win a majority when votes are cast on may 7 in what is being touted as the closest and most unpredictable race since the 1970s .
prime minister david cameron , of the conservatives , narrowly topped the polls assessing who ` won ' the two-hour clash against six other parties in an unusual format for britain .
crucially , 33 per cent of the 7.4 million viewers hailed cameron as ` the most impressive ' on the subject of the economy , which lies at the heart of each party 's campaign .