Chevaline
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The Chevaline project was a secret project to upgrade the British Polaris missile system. The exact length and cost of the project is unknown, but it may have started before Polaris became operational in 1968 [1], and was certainly started by 1973 [2]. Test launches were conducted by 1977 [3], but the project was unknown until a government announcement in 1979. This announcement was forced out of the government because the massive cost overruns were now so large that the project was impossible to hide anymore.
Technical
The need to upgrade the system came about because of the Soviet deployment around Moscow of the Galosh ABM system. The philosophy of Britain's nuclear deterrent was not the MAD philosophy of the USA and USSR, but was to offer in response to an attack, guaranteed destruction of Moscow. (The philosophy never explicitly stated Moscow as a target, but the technology of the time meant that weapon targeting was determined in advance, and could not be changed quickly). By the late 1960s, simple calculations showed that the probability of achieving this destruction was not likely, and with realistic system failures included, downright unlikely.
The response was to reset to the equation to the pre-Galosh state. Other solutions such as upgrading to the Poseidon system, increasing submarine availability or launcher reliability were either not practical, or thought too expensive. Political considerations at this time in the UK were secondary because of the thick veils of secrecy around the nuclear establishment.
The solution was seen to be two fold, increase the number warheads carried by the Polaris system from one to two, and increase the likelihood of warhead survival. This was to be core of the Chevaline project.
The project involved building a warhead and penetration aid carrier called the PAC, which held two hardened RVs, and a series of deployable penetration aids. The carrier could also maneuver to help confuse the Galosh fire control radars. Although maneuverable the system was not a MIRVing system because the targets of both warheads were effectively the same.
The project was carried out under extreme secrecy by a team consisting of the Atomic Weapons Establishment (AWE) at Aldermaston, the Royal Aircraft Establishment (RAE) at Farnborough, and Hunting Engineering at Ampthill. The system was tested at the US Eastern Test Range, Cape Canaveral, and the warheads were tested with underground tests in the Nevada Desert, indicating that there was significant US knowledge and cooperation in the project.
The integrated upgraded Polaris system was called A3TK. This system was in service from 1982 to 1996, when it was replaced by Trident.
Political
The Chevaline project was kept secret by a succession of UK governments, both Labour and Conservative. This included the governments of Harold Wilson, Edward Heath, and James Callaghan. The project was finally revealed, but not cancelled by Margaret Thatcher's then defence minister Francis Pym. The reasons for revelation were both political and practical. The cost over runs of the project were now so enormous (greater than £1billion in 1979) that the secret inner cabinet spending approvals could not continue. At the same time Thatcher saw a way to score political points off her outgoing Labour rival Jim Callaghan. In no way was Thatcher anti-nuclear, and she made no attempt to terminate the project. In fact later in her Prime Ministership she approved the purchase of the hugely expensive, and vastly more powerful Trident system.
External references
[1] http://www.skomer.u-net.com/projects/chevaline.htm
[2] http://www.awe.co.uk/main_site/about_awe/keeping_the_peace/1973.htm
[3] http://www.astronautix.com/lvs/polrisa3.htm
[4] http://www.fas.org/nuke/guide/uk/slbm/chevaline.html
Chevaline is also the name of a commune in the Haute-Savoie département of France.