The 'one nation' line was not just 'penned for him by some scriptwriter'.
It was a direct reference to Benjamin Disraeli, who famously used it, and who was a Tory. It is intended to show that the party that is interested in governing for the WHOLE country and not just for the 'elite' is now the Labour party.
And coming a week before the Tory party conference, it is a very potent line to take. They somehow have to prove they are not just on the side of millionaires (who they are giving £££ of tax cuts to) but also on the side of those on housing benefit, the disabled, the vulnerable and the poor. But to those people they cut their benefits and say 'get back to work'. Never mind the lack of jobs, or the sheer impossibility of going back to work.
I thought Ed did well. The proof of the pudding is in the eating, and what will be the decider for Ed is what their manifesto at the next election is. The questions to ask will be 'what is their plan', 'what does it cost' and 'will it work'.
It is not possible for them to do that yet, because who knows what a dog's breakfast Gideon can make of the UK economy between now and then? But the promises made for the next election from all parties are the things to judge them on.