Despite its name the National Portrait Gallery does not actually represent the nation. It represents an elite, those deemed to have shaped Britain’s past or held to illuminate its present: kings and queens, statesmen and diplomats, poets and artists,. It is a gallery of the great and the good, the Dictionary of National Biography in museum form. This bias goes back to the institution’s nineteenth-century origins and continues to reflectthe Victorian values which brought it into being. As Lord Palmerston, Prime Minister of the day, proclaimed to Parliament: “There cannot be a greater incentive to mental exertion, to noble actions, to good conduct on the part of the living than for them to see before them the features of those who have done things which are worthy of our admiration, and whose example we are more induced to imitate when they are brought before us in the visible and tangible form of portraits...”

The NPG’s peculiar historical status as a museum of inspiring examples has led to the development of a problematic anomaly in the balance of its collections. Like any other museum it constantly conducts research into the paintings that it possesses, which means inevitably that previous attributions sometimes evaporate. When they concern the artist, no great adjustment is necessary: Charles II, by Sir Peter Lely becomes Charles II, by Anon, and the label is altered accordingly. But when they concern the sitter, the consequences are potentially much more damaging. If Richard, Third Duke of Cumberland, by Thomas Gainsborough should suddenly become Unknown Gentleman, bythe same painter, then the rules of the museum insist the picture is instantly removed to storage until such time as an alternative identity should be established. This applies regardless of how good painting in question might be.

As a result, a subterranean sub-collection of unexhibitable portraits has gradually built up in the NPG’s basement Each picture was bought in the belief that it represented a particular individual, but now that supposed identity has been disputed or disproved. At the last count, there were more than 150 such pictures in the NPG’s vaults. So what to do with this collection of malingerers and outsiders, this rogues’ gallery of unknowns? Tarnya Cooper, the NPG’s curator of sixteenth-century paintings, has had the enterprising idea of making an exhibition from the holdings in the vaults – although, given the rules, it cannot take place in the museum for which she actually works.

Hence “Imagined Lives: Mystery Portraits c.1520-1640” an enthralling display of works borrowed from the NPG’s basement and currently to be seen in the long gallery of the National Trust’s Montacute House, a real gem of Elizabethan architecture in Somerset. The selected pictures turn out to be fascinating and evasive in equal measure. Meet the blushing unnamed Elizabethan courtier, flushed by inscrutable passion for an equally unknown woman. Meet his appealing but distinctly dodgy contemporary, a man with a fake coat of arms, an impossibly spotless lace ruff and a distinct twinkle in his eye: the epitome of the Shakespearean bounder, he holds out a pair of carnations, symbols of Love, in his attempt to fool some gullible young noblewoman into marrying a scoundrel beneath her status. Meet the soft-skinned, floppy-haired, sensitive Stuart courtier, expiring delicately on his deathbed. Meet a whole crowd of other vibrant Anons, whether pale or ruddy of face, sleek or gaunt, self-assured or morbidly watchful.

The precise identities of these men and women may have been forgotten or obscured, but their portraits are no less fascinating for that, and while the loss of their names means that they no longer fit into some old-fashioned, kings-and-queens, textbook version of history, they are nonetheless part of history in the greater and truer sense – just as surely as generations of now-forgotten gentlemen-farmers, labourers, publicans, butchers, bakers and candlestick-makers are part of history too. This might be a modest little show, but it raises some large questions. Perhaps it is about time the National Portrait Gallery changed its rather rigid approach to identity and importance and let at least some of the unknowns and also-rans – those quiet but continual contributors to the fabric of life – into its rather fusty and dusty picture of the nation’s past.