Tom Stoppard and Andre Previn’s 1977 collaboration, billed as a play for actors and orchestra, originally premiered at the Royal Festival Hall, which, therefore, inevitably put the emphasis on the music making around the brief, but dense and compelling portrait of two patients at a mental hospital - one who is really ill, imagining that he is surrounded by a live orchestra, and the other, a political dissident who has been incarcerated for his opinions. The latter is in a double bind - he can only be released by acknowledging that he was ill and has been cured, but in refusing to do so, he will continue to be held captive.

Now it has moved further along the river to the National, where the young professional orchestra Southbank Sinfonia provide the living (and musical) embodiment of the patient’s imagination, but the play steps beyond their shadow to emerge forcefully as more than just a footnote to Stoppard’s career, but a vivid piece of political theatre in its own right.

At just over one hour’s running time, it is both intense and audacious, and in Felix Barrett and Tom Morris’ production that puts the orchestra on a revolve and frequently has them stepping into and out of the action, it packs a powerful punch.

In a programme note, Stoppard points out that it may, sadly, not have lost its resonance and relevance, either - he quotes that The Times reported last year that a parliamentary candidate for a political party in Russia was sent to a psychiatric hospital after police questioned him about his political activities.

The fact that it may be the lunatics who are running the asylum is neatly articulated by Dan Stevens’ very funny turn as the doctor, while Toby Jones brings his wayward eccentricity to the role of Ivanov and Joseph Millson’s gauntly physical turn as dissident Alexander, provide moving contrasts in the fine line between the mental territories they occupy.