You can see why some people think George Bernard Shaw is as likely to come back into fashion as bread and dripping. He can be a prolix, hortatory old bore. The prefaces to some plays run to 60 densely argued pages, covering such urgent topics as the need for phonetic reform of the English alphabet.

Yet, just as you’re not waiting for another Shaw, two come along at once – and demand a reassessment, of sorts. Peter Hall’s new Pygmalion is a sunny delight (see opposite), and Marianne Elliott’s revived Saint Joan is a valiant, ambitious attempt at what many regard as his finest play. Even though Elliott has made judicious cuts, it remains long and tremendously wordy. And though few can dramatise a purely intellectual debate so deftly and wittily, three hours is still a long time for a play with such little sense of narrative movement, and more false endings than The Lord of the Rings.

The most admirable thing about Shaw’s foray into the medieval world of Jeanne d’Arc is his even-handedness. The churchmen here assembled are shown to be averagely good, merciful men, desperate to save both Joan’s earthly life and her immortal soul, though the eradication of heresy is ultimately more important to them. The Maid’s trial is shown to be remarkably fair: she is given every opportunity to repent and be pardoned. The punishment of death by fire is certainly cruel by our standards; but, as Shaw well understood, they didn’t live by our standards. “If you are quite convinced,” he lectures in his preface, “that the world has progressed enormously, both morally and mechanically, since Joan’s time, then you will never understand why Joan was burnt . . . and until you feel that, you know nothing essential about her.” Shaw’s great achievement is to bring the debates and concerns of the Middle Ages to life without condescension, and to involve us in them with startling immediacy.

A central problem is Anne-Marie Duff’s Joan. Her strange, unplaceable accent is okay: somewhere between Bantry Bay and Derry Quay, Galway and Dublin town. Her look is convincing: a sallow, underfed peasant from the backwoods of Lorraine. But, at the risk of stating the bleedin’ obvious, Joan of Arc must have been a girl with considerable charisma. Anyone acting her must exude that – and Duff doesn’t. She’s a manic shrimp, energetic, excitable, but not inspiring. She suggests a certain steeliness, but not that maniac-fundamentalist fire that had hard-bitten French soldiers following Joan to their deaths on the walls of Orléans.

The soldiers, too, are unconvincing. You want blustering thugs roaring around with ugly spiked maces and the clump of boots on wooden boards. This lot wouldn’t frighten anyone. I know they’re meant to be French soldiers, but even so. In the same vein, there are no decent battles. Instead, they are balletic and metaphorical. The titanic siege scene boasts powerful percussive rhythms, which work well until hopelessly undermined by men in vests advancing across the stage towards us, banging chairs on the ground. I don’t get it.

Then again, Shaw is primarily a dramatist of ideas. And the lengthy debate here between the urbane Richard, Earl of Warwick (a delightful Angus Wright), the passionately intelligent Bishop Cauchon of Beauvais (a superb Paterson Joseph) and Chaplain de Stogumber (Michael Thomas) is brilliantly done. The three men range over individualism versus collectivism, the decline of feudalism and the Catholic church, the rise of nationalism and Protestantism (wildly anachronistic – Shaw at his most mischievous) and the pros and cons of Islam (topical), and it is cogent and compelling throughout. The cause of their meeting, to decide the fate of Joan, becomes rather peripheral. An odder performance, enjoyable in small doses, is Paul Ready as Charles the Dauphin, played as a petulant boy slopping around in his dressing gown.

There is some wonderful music from an ensemble tootling away on woodwind and psaltery, and some atmospheric Gothic Voices-style singing. The overall design is grimly washed out, though. The set is a bare, sloped platform and some wooden chairs, against a background of splintered tree trunks reminiscent of Flanders circa 1917. The lighting is no more than harsh white spotlights, which at their best add intensity, but at worst feel more like a punishment.

All in all, Elliott’s new Joan is a qualified success; and, along with Hall’s Pygmalion, it suggests how Shaw can be made to work for a modern audience. The old fox could also teach young writers how to get away with whole rafts of social and political hectoring, by spicing it heavily with such sprightly intelligence and wit.

Saint Joan

Olivier, National, SE1

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