Work on the first new exhibition spaces the nation’s most important paintings to be created in more than 30 years is set to get under way as part of a long-awaited overhaul of the Scottish National Gallery in Edinburgh. However the cost of the project – which also involves a new look for part of East Princes Street Gardens – has soared by nearly a third, is running three years late, and has had to be scaled back significantly since plans were first unveiled four years ago.