Cllr Mike Pavey, Deputy Leader of Brent Council, has an interesting column here. He is quite right that ceding economic credibility is incredibly damaging to the Labour Party's electability. It has also led directly to the damaging and unneeded austerity that we have suffered since the Tories came to power in 2010.
Always demanding that the government spend more (as many on the left do) actually undermines the case for counter cyclical spending when it is needed. Whereas Mikey seems to feel that this is a past debate, I think it may well come back fairly shortly. UK inflation is now more or less zero. If events in China lead to falling oil and commodity prices, that may well lead to an extended period of deflation. The classic Keynesian response to that would be a fiscal stimulus to raise prices in order to avoid a deflationary spiral. If that is conflated with sheer extravagance of simply spending to no great purpose (such as simply nationalising everything), I suspect that many voters will find it incredible and adopt George Osborne style cuts instead.