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Nick Robinson shines a light on the process by which controversial decisions are reached behind closed doors in Westminster and Whitehall.

In this programme, he discusses whether to spend billions of pounds building four new submarines to carry the United Kingdom's nuclear weapons.

Trident has to be renewed and renewed soon, we are told, because the subs being used now will simply wear out.

Is the nuclear deterrent the ultimate insurance policy in a dangerously unpredictable world or a relic of the cold war, an unusable weapon which sucks billions of pounds away not just from public services but from combatting today's threats - terrorism, cyber attacks and drone warfare.

To discuss the issue, Nick is joined by the two men who until a few months ago were responsible for this decision: Dr. Liam Fox, the former Conservative Defence Secretary and committed supporter of Trident, and his deputy, Sir Nick Harvey, a Liberal Democrat who set up the process of looking for an alternative.

With them are Professor Malcolm Chalmers, special adviser to two Labour Foreign Secretaries, and now research director at the Royal United Services Institute; Sir Richard Mottram, former Permanent Secretary at the Ministry of Defence who was chairman of the Joint Intelligence Committee after the Iraq war; and Lord Ramsbotham - formerly General Sir David Ramsbotham - Commander of the British Field Army who has described Trident as a virtual irrelevance.