Why is Labour’s welfare policy such a mess? The party’s shadow work and pensions secretary, Margaret Greenwood, has today announced a year-long review into the government’s troubled universal credit programme in an interview with the Mirror. The interview is part of a wider “reset” of party policy on welfare, but it effectively means that three years into Jeremy Corbyn’s leadership the party has returned to the point it reached when Rachel Reeves was welfare lead under Ed Miliband: waiting to see how universal credit turns out.

But that position was a lot more defensible in 2015, when universal credit was still largely unimplemented. Now the roll-out is happening in earnest, we can see that UC is not working well and that it either needs to be substantially reworked (which requires a lot more money to be put into it) or scrapped and replaced (which again, is going to require a lot more money to be injected into the welfare system).

Greenwood’s name is often kicked about when Labour party staffers, whether Corbynite or Corbynsceptic, list members of the Shadow Cabinet who they feel are underperforming. But she is the victim, rather than the architect, of the party’s policy mess.

It’s true to say that Greenwood has a profile more commonly associated with people in witness protection than the opposition front bench, but the blame for that should be shared roughly equally between two people: Debbie Abrahams, her predecessor as shadow secretary of state for work and pensions; and Jeremy Corbyn.

The biggest problem that Greenwood has is that at the last election, the shadow ministers in charge of the other two big spending departments (Health and Education) went into meetings about manifesto planning with clear spending asks: Jon Ashworth at Health simply wanted to be able to say Labour was spending more than the Tories on the NHS, while Angela Rayner had a swathe of policies she wanted funded alongside the leadership’s pet project of scrapping tuition fees.

Abrahams made no such demands, and as a result there is no spare money in the party’s tax-and-spend plans to do anything about the ongoing car crash that is universal credit. That leaves Greenwood in the impossible position of opposing the day-to-day practice of universal credit … while having no money to actually do anything about it.

While a large share of the blame fairly accrues to Abrahams, an equally large portion must go to Jeremy Corbyn, who kept an underperforming Abrahams in post long past the point where he could have traded up for a better option and only removed her after she faced credible allegations of bullying.

The only politician who can fix the mess now is John McDonnell. If the shadow welfare team is going to break out of witness protection they are going to need a lot of hypothetical money to do so – and the only person who can sign off a big revenue-raising measure is the Shadow Treasury team. And until that happens, Labour’s welfare policy will continue to be a mess.