THE UK has had at least one nuclear-armed submarine somewhere at sea for the past 40 years.

For some it represents an essential deterrent, the ultimate insurance policy, protection from an all-out attack wherever it may come from. It also guarantees Britain a seat at the UN Security Council, putting the country at the heart of global diplomacy.

For others it offers no reassurance at all. For them, our nuclear threat is quite simply an obscenity, morally unjustifiable and financially ruinous. Far from projecting power, the US-bought, US-maintained Trident ties Westminster uncomfortably close to Washington.

The LibDem-led report on the future of Trident, the UK's present nuclear weapons system, was not part of the debate between those two irreconcilable points of view. The Coalition split it has prompted is neither here nor there as far as those who reject the very idea of nuclear weapons are concerned. The Conservatives back a like-for-like replacement of Trident, which means four subs. The LibDems favour a slightly reduced level of readiness – three subs in other words.

Those opposed to nuclear weapons on principle could be forgiven for thinking, some split.

Where the report does have real merit, however, is to introduce some useful shades of grey into a debate that is usually presented as black and white. Here the comments of Chief Secretary the Treasury Danny Alexander, who was responsible for the Trident Alternatives Review, are worth considering.

He noted the £4 billion saving that moving from a four-sub fleet to three would generate. But the real importance of a new approach, he said, was as a spur to multilateral disarmament. The move would allow Britain to "contribute meaningfully" to global efforts to reduce and eventually remove nuclear weapons across the globe.

Mr Alexander is surely right to remind us that "supporters" of nuclear weapons would like to see the back of them too. His way forward is a step in the right direction – if, that is, you believe some level of nuclear deterrent remains vital for national security in a post-Cold War but still uncertain world. And if you believe a smaller fleet of nuclear subs can adequately provide such protection.

That is the debate the Conservatives, their LibDem partners in Government and the Labour opposition at Westminster will have ahead of the 2015 General Election and the final decision required in 2016. In Scotland, though, the discussion will be wider and more urgent.

The staunchly anti-nuclear SNP has vowed to remove Trident from the Clyde as promptly as is safely possible if Scotland becomes independent. Yesterday its defence spokesman, Angus Robertson, dismissed the Government review as "not worth the paper it is printed on". Patrick Harvie, the leader of the Scottish Greens and an ally in the Yes Scotland campaign, said it was "fundamentally flawed" and the row over three or four subs "meaningless". The issue will feature prominently in the run up to next year's referendum.

So it should. Scotland benefits from the many jobs that come with a major naval base but lives with the potential risks of housing Britain's missile stockpile. As the debate over the future of Trident began afresh yesterday, it is right that Scotland takes the lead.