One of the first atomic bomb stores built in England has joined a bell foundry, a fairground rollercoaster and a tumble of stones which may once have been a monks’ kitchen among more than 5,800 important listed buildings and structures which appear on the latest English Heritage at-risk register.

Although thousands of buildings have been rescued since the register was first launched in 1998, the list is growing faster than buildings are being saved, a situation worsened by government cuts to English Heritage’s funding. In the past year, 318 sites have been saved and removed from the register, but 360 new sites have been added.

Many make the list because their original purpose has vanished – fortunately in the case of the scruffy sheds and towers at Thetford Heath in Suffolk which was one of Britain’s first purpose built stores for atomic bombs. They were top secret when built in the early 1950s to house the RAF’s first atomic bomb, codenamed Blue Danube, using a special grit-free surface to prevent the risk of sparks. The explosive components were stored separately in deep thick walled pits, and ringed with watch towers, triple wire security fences and guard posts. They were decommissioned within a decade, and no longer secret, soon targeted by vandals. They are now listed at Grade II*, the second highest category, recognising their importance in the history of Britain and the cold war.

The current owners have worked with English Heritage to repair the watch towers and the kiosks which housed the nuclear components, but much remains to be done, and even the most ardent conservationist would find it hard to dream up a new use for such a site — the problem for the most intractable cases on the at-risk register, including old industrial sites, hospitals and other sites which have lost their original function and have no obvious economically viable new role which could pay for the restoration work.

Just 13% of the 5,831 buildings and sites on the register are considered economic to repair. For the rest – such as The Monks’ Oven, a heap of stones in a green field in Cumbria believed to be part of a medieval monastery – the total gap between the cost of restoration and the amount they could ever earn is now estimated at £423m, up from £330m in 2007, an average of £366,000 per site. Over the same period government funding for English Heritage has repeatedly been slashed, so that its grants budget has been cut in turn by 40% in real terms.

That dismal picture is about to worsen, as English Heritage appeals for volunteers and local societies and groups to help survey the condition of England’s 385,000 Grade II listed buildings, which make up 92% of all listed buildings. Outside London these have never been included in the annual at-risk surveys, because the scale of the task was beyond English Heritage resources. If the guesstimate that at least 5% of such buildings are in trouble, next year’s at risk register will swell dramatically.

A pilot project has already been launched in Lincolnshire, where Heritage Lincolnshire has almost completed the survey: preliminary results suggest that 6.1% of the Grade II buildings are at risk. One man, retired accountant Graham Umpleby, from Boston, has single-handedly collected most of the bad news, scrambling through nettles and over tumbledown walls to complete surveys of more than 9,000 sites.

Simon Thurley, chief executive of English Heritage, said the Grade II buildings were of enormous importance to their areas. “When one of them is lost, it’s as though someone has rubbed out a bit of the past; something that made your street or your village special will have gone.”

There is better news for some sites. One of the apparently hopeless cases, the burned out shell of the 1919 wooden Scenic Railway roller coaster in Margate, the first fairground ride to win Grade II* listed status, has won a lottery grant and with the help of a local trust is planned as the centrepiece of a restored Dreamland funfair. Repairs are almost complete at the crescent at Buxton, once the glory of the Derbyshire spa town.

The buildings of Taylor’s Bell Foundry in Loughborough, which in 1851 cast one of the largest bells in the world, Great Paul for St Paul’s cathedral, appear on the list for the first time this year. The buildings are Victorian, but the most recent in a line of bellfounders dating back to the 14th century. The firm was rescued from administration in 2009, but the buildings – the only purpose built bell foundry in England – have suffered from generations of neglect or inappropriate repairs. English Heritage has offered a project development grant, and there are plans for restoration and more public access to the site.

Edward Impey, director of heritage protection at English Heritage, said: “There has never been a time when imagination and commitment has been more important.”