The stereotypical image of the English Civil War is that of clumsy Roundheads and graceful Cavaliers – when I think of this period, it’s Keeping Up Appearance‘s pageant, in which Hyacinth Bucket’s slovenly brother-in-law is deemed a perfect Roundhead, that comes to mind. In Howard Brenton’s densely written play, however, the more aesthetically pleasing Charles Stuart is the poster boy. A romantic figure with flowing auburn locks and collar of Antwerp lace, he signifies an idealised past. His antagonist Oliver Cromwell, meanwhile,  attempts to forge a new British identity by trying a king through the legal system, finding him guilty and forming the only British republic in history. Taking place over December and January 1648-9 in the weary, disillusioned aftermath of the Civil War, Cromwell rises up the ranks and the astonishing prospect of putting the king on trial for treason becomes ever more realistic. We know how the story will end; the crucial thing is how it unfurls.

Howard Davies’s slick production has an unusual tone that gives the impression of watching a modern interpretation of a Schiller play. The subdued colour palette, be-suited politicians and pounding of typewriters (designed by Ashley Martin-Davis) have a 1930s feel – an era that’s far more graspable than the 17th century but still not our own. Charles is the only character to wear 17th century costume (a similar device to that used by Phyllida Lloyd in her production of Mary Stuart), as if to signify that he is anachronistic and out of touch with his subjects. The traverse staging seems to invite the audience to take sides – neither of which are easy for contemporary audiences to empathise with, as there is fanaticism on both sides.

Following on from his turn as a comedy Cavalier in the Restoration comedy The Recruiting Officer earlier this year, Mark Gatiss gives a finely tuned performance as Charles. Effete and eternally glib, he treats his trial like a charade, and Cromwell’s proposed compromise of constitutional monarchy with the king as a figurehead (which could have saved his life) is deemed a frightful joke.

Douglas Henshall is quietly compelling as the “so practical yet so damn religious” Cromwell who believed himself to be God’s mouthpiece, perfectly embodying his brooding internal conflict. Amongst the large supporting cast, Tom Vaughan-Lawlor is memorable as John Cooke, the nervous lawyer with radical leanings given the enormous job of head of the prosecution when all other lawyers in London conveniently disappear.

Inventing confrontation scenes between the key players is a dramatist’s prerogative and Brenton’s imaginary meeting between Charles and Cromwell invites comparisons with Schiller’s confrontation scene between Mary Stuart (Charles’s grandmother) and Elizabeth I. In Cromwell’s attempt to save Charles rather than destroy him, we see a peculiar kind of Stockholm syndrome, possibly because both believed that they were uniquely singled out by God.

55 Days doesn’t have the salty flamboyance of Brenton’s recent Globe hit Anne Boleyn and features several laborious passages that would require careful re-reading and thorough research to fully grasp. At its most stirring, though, it’s an intelligent breath of fresh air in this year of sickening sycophancy towards the monarchy, reminding us of the extraordinary way in which this group of rebels were able to prove that a king could commit treason against himself.

55 Days plays at Hampstead Theatre until 24 November 2012. For more information and tickets, visit the Hampstead Theatre website.