So what happened to the money?
I think the key issue on these savings that is seldom asked is ... 'What happens to the money'. Public sector financing is such that a department gets a big chunk of money each year and then spends it as it sees fit. GDS has some control over any of this spend that falls into IT. BUT (and here is the key bit) if GDS force a reduction in scope and reduce the costs of a programme .... what happens to the saved money... It doesn't go back to HMT (for such an act would be an admission from a department that they asked for too much, and impact budgets next year!), rather it is reallocated to AN Other IT project or area of spend. All departments run huge portfolios of projects and there is always a need for money. So the money isn't 'saved' at all.
I think rather than bemoaning a lack of evidence, the NAO should call Cabinet Office out and investigate the actual value being added by these layers of bureaucracy.