How would define what is in the interest of the tax payer? Is it longer term gains and benefits or short term costs & savings ?
The classic example being our aircraft carriers, a 2bn cost change was nixed by MoD civil servants because the short term cost made it seem they'd lost control on budgets, yet the long term benefit, none of which the decision makers would be around to get reward from, would have costed in and been much better for the taxpayer.
Fwiw my impression is Infosys have cornered the market in terms of offering the right balance between skills & costs, so you actually get value out of engaging with them as subcontractors, rather than their rivals like Accenture, expensive, or TCS, constant churn of staff.
So they win contracts because they seem to know what they're doing and the offer is decent, not because of who they know.