After months of speculation, the government will soon lay out plans to change the benefits system. Keir Starmer argues that the current system is ‘the worst of all worlds’. But with deep cuts to disability payments on the table, could the changes come at the expense of the most vulnerable? And will Labour MPs really be able to support this? John Harris hears from the head of social policy at the New Economics Foundation, Tom Pollard, and the Guardian columnist Gaby Hinsliff