The 74 manifesto was the most radical (with possible exception of 45) that Labour has ever fought an election on.
OK, they weren't able to hold them to implementing it - that is unarguable. And Benn failed by telling the left he was making his case privately around the cabinet table [which to be fair he was] rather than mobilising for democratic reforms to the party when in government. But you can imagine the pressure ("you're recking a Labour government and helping the Tories to gain power"), so he waited till after the election and the right discredited itself. Not that they cared, they fucked off and formed the SDP and helped Thatcher stay in for 18 years.