Emma Barnett was very clever (and underhand) in using the opportunity, when Corbyn was trying to find the costings, to stick in the barbed reference to Gordon Brown.
Corbyn: "Can I give you the exact figure in a moment?"
Barnett: "Is this not exactly the issue with people and the Labour Party which came up under Gordon Brown that we cannot trust you with our money?"
That's a cheap shot right there. The guy hadn't memorised the figure, which she had already been given earlier by Angela Rayner. The issue, as Emma well knew, was that Corbyn hadn't memorised the figure. That was it. Nothing about the rights or wrongs of the policy and how it would effect the lives of the Womens Hour audience. She made it an issue about memory to throw Corbyn off balance and took the opportunity to get into the audience's mind, almost subliminally, her own message - that Labour are not to be trusted with our money.