"THIS is strange for us as well" says Rear Admiral John Weale, as he prepares to open the hatch, so to speak, of one of the UK's four nuclear submarines to around two dozen reporters, broadcasters and photographers.

The submarine element of the Royal Navy is known as 'the silent service'. The nuclear deterrent, some of the 160 or so HMS Vigilant crew members tell us, is so sensitive that only a handful of them know where in the world they are when on patrol. "It's just a bit of sea wherever we are" says an officer, explaining that the information is shared on a strictly need-to-know basis.

HeraldScotland: A member of the armed services walks on the deck of Vanguard-class submarine HMS Vigilant

A large media event, then, at the UK's nuclear submarines' bleak Faslane home teeming with checkpoints, armed guards and barbed wire, seems counterintuitive. But these are not normal times for Trident, with a key vote on renewal of a British nuclear deterrent to take place this year and UK Labour under Jeremy Corbyn poised to break a consensus between Government and opposition for the first time since the early 1980s. The circumstances explain the Royal Navy throwing open its nuclear deterrent to the glare of publicity for the first time in a decade and the offer of unprecedented access to staff and mechanics of HMS Vigilant, which was launched in 1995.

But while the access to the submarine's weapons control centres, missile rooms and even its nuclear reactor - viewed through a thick block of glass which we are told the only window on board - are not off limits, it is the personal stories of the crew that proves most fascinating.

Submariners all put themselves forward to serve on the underwater vessels, which in the case of Trident submarines means deployments potentially in excess of three months or more without seeing daylight or breathing fresh air.

Contact with their families is limited to one note per week, a maximum of 120 words. They will be read by several people before being passed to submariners, who cannot reply, with one now legendary message reading simply 'one of us is pregnant' two weeks into a patrol.

While Trident subs can receive messages, they cannot be sent out as doing so could disclose the deterrent's location to enemies. The decision to break radio silence can only be taken by Dan Martyn, HMS Vigilant’s commanding officer, and even a death on board, which have occurred, may not be enough. He would "retain or return" should the situation arise, meaning he would break cover or continue at sea with a body on board depending on circumstances.

He says it is dilemmas like these - rather than authorising a nuclear strike - that play on his mind. "You can't take command of one of these things without thinking about the worst case scenario," he adds, after being asked about how he feels about his potential role in a live launch. "I go to sea and I worry about things but to be honest what I tend to worry about is the health and wellbeing of my ship's company. What keeps me awake at night will be that decision of whether to break radio silence."

But in the event of a live launch, it would be Cdr Martyn that would give the final OK, after two crew members decode a message and he authenticates it. No one crew member could authorise a strike alone, he says, with even the 'letter of last resort' from the Prime Minister, containing orders should the UK Government ever be destroyed, to be opened in the company of another.

HeraldScotland: Undated handout file photo issued by the MoD of the test firing of a Trident missile, as Jeremy Corbyn was still struggling to draw a line under his reshuffle after three shadow cabinet members refused to rule out quitting if Labour drops its backing for

"There's no great secret, it's in that safe over there," he says, darting over to a box with a simple dial on the front in the command centre that, like much on board, appears dated. "The way I've resolved this with myself is if I've received a requirement from the Prime Minister to fire, I'd operate in accordance with my documentation, and return to a position of deterrence."

Throughout the submarine, occasional pictures of the Queen and plaques from other ships appear. Officers have their own small bunks - three to a room - while trainees sleep metres from the nuclear warheads, which are kept in sixteen huge vertical tubes although only eight missiles, with up to 40 operational warheads split between them, will remain on board at once.

When not working or sleeping, trainees play computer games with the FIFA football simulation the most popular. There is a rowing machine and treadmill, DVDs and well-thumbed magazines in mess areas. Cdr Martyn allows his company to drink alcohol, although it is strictly rationed at a two can a day limit.

Medical treatment is provided by a single, on-board doctor and a small team of medics in a small sick bay. "It's a unique opportunity and challenge, particularly medically," says Dr Tweed, who previously worked in an NHS A&E before joining the Navy. "It's completely different, there's not as many drunk people around, that's quite nice."