I think the real issue is that the MoD has very little "control" over spending in the sense someone like Nissan or say Marks & Spencer have. They are tied into long term programmes that are managed by the contractors and if the costs overrun they only really find out when the bill (weekly/monthly/yearly) comes in - and it has to be paid whatever
The dependence of "savings" is really a measure of how much the predicted costs exceed the funds available TBH
The only answer is more money or fewer major programmes