Never exhibited before in Britain, the collection includes images of Charlie Chaplin directing Limelight, and stills from films including Rebel Without A Cause, The Seven Year Itch and Orson Welles's The Trial.
Among the most striking images is one taken by Jewish photographer Eve Arnold of Marilyn Monroe practising her lines in the Nevada desert ahead of a scene with Clark Gable in The Misfits. Arnold's original Rolleiflex camera will also go on show alongside scripts, costumes and props from dozens of movies.
Hardcastle, who describes the team working on the project as "like a commando group", stresses that the Covent Garden branch of the LFM is not really a museum. "Visitors will be part of a conversation here rather than just hearing a lot of noises. It is not a museum in a true sense. Museums do not change, but we can adapt everything.
"What Jonathan has done proves what can happen with enthusiasm and talent."
Among the obstacles to overcome in the renovation of the site were the requirement for specialist air-conditioning and temperature controls to protect the exhibits, and the obvious need for security provisions. Such has been the scale of the rebuilding that flat screens will be erected to show visitors just how much work has gone into the project.
Revolutionary technological methods will also be employed. Film fans will be handed iPads with specially-developed apps as they enter. At each image they can scan codes to view in-depth information on the films, actors and directors. The iPads will show trailers, clips from the movie in question, and more stills. "To have this interaction with an exhibition and to mix the platforms in this way means we are really giving visitors a new experience. We are joining the dots for them," says Sands.
The contemporary design of the cellar means Sands and his team can adapt the space to the exact requirements of any future exhibition. As well as the main display space, there are side-rooms for smaller exhibits, halls suitable for hosting launches or corporate functions, and the obligatory café and gift shop.
Sands bubbles with all the enthusiasm you would expect of a film buff let loose in a cinematic sweet shop. One suspects he feels like pinching himself from time to time - he has the bonus of turning up to work and finding boyhood heroes and legends of the industry grafting alongside him.
In addition to Hardcastle, veterans such as Linda Ayton - legendary director Fred Zimmerman's former assistant - have been involved in the project.
The Magnum exhibition represents a return to Sands's first love - he began his career as a stills photographer at Elstree Studios.
"When I walked round our City Hall museum I thought to myself that, if I had my time again, I could do this and specialise that. Technically that's what we have done here.
"My commercial partner, Paul Mendoza, said I should pay homage to my roots. I know the subject well, and so that passion has really helped me push this project.
"We are quite humbled by the amount of support that we have generated from the film companies, from the British Film Institute, and from the National Media Museum in Bradford.
"We have almost created a vault. We have pieces including Charlie Chaplin's trademark bowler hat and cane, original Planet of the Apes suits, and Christopher Reeve's Superman costume."
Sands is hoping that 100,000 visitors a year will come through the doors. If that happens, it would be, for him, a very happy Hollywood ending.
'Magnum on Set' is at the London Film Museum at Covent Garden from tomorrow until September 1. www.londonfilmmuseum.com
