The four Vanguard-class submarines form the UK's strategic nuclear deterrent force operating today.

All the four boats - Vanguard, Victorious, Vigilant and Vengeance – were built here in Barrow and each is equipped with Trident II D5 nuclear armed missiles.

HMS Vanguard was the first of the four submarines of her class.

She carried out her first Trident missile test firing in May 1994 and the first of her operational deterrent patrols in 1995.

HMS Vanguard is currently in Portsmouth undergoing maintenance and sea trials.

Like the earlier Resolution class, all Vanguard boats are based at HM Naval Base, Clyde.

An idea of the care and precision needed to provide modern submariners with a comfortable working environment is provided by the official commissioning brochure for HMS Vigilant on November 2 in 1996.

It noted: "The key to a credible nuclear deterrent is the submarines' endurance and its ability to stay dived at sea for very long periods.

"One facet of this endurance is the independently maintained submarine atmosphere.

"Oxygen is produced from the hydrolysis of water, carbon dioxide is removed by amine filled scrubbers, dust and aerosols are removed by charcoal filters and temperature and humidity are controlled by air-conditioning units."

A similar brochure for the commissioning of HMS Victorious on January 7 in 1995 noted how a team of chefs made sure that the food at sea was of good quality - and variety.

It was noted that during the contractor's sea trials you could have chosen from a menu which included lentil and barley soup, ham risotto beef Wellington, lobster Americane, king prawns or Strudel and fresh cream.

The boat also needed to be ready to welcome guests when not on patrol.

It noted: "HMS Victorious can expect to entertain many formal visitors in a variety of ways."