The Magna Carta Plays (Salisbury Playhouse)

Four playwrights were invited to respond to the 800th anniversary of Magna Carta in a city where the cathedral holds the best preserved copy of the most famous legal document in the world. Howard Brenton's snappy thriller Ransomed hinges on the theft of it from "Melchester" where the canon is self-flagellating and the spire is collapsing.

None of the other plays – Kingmakers by Anders Lustgarten, Pink Gin by Sally Woodcock and We Sell Right by Timberlake Wertenbaker – seem so immediate, though all have their virtues, and all are more interesting than the suggestions for a new clause elicited from the audience tweeting on a front cloth: "Children can choose what time to go to bed"; "animals should be treated as sentient beings."

But there's also a disappointing unanimity of approach despite contrasting settings: the charter was a whitewash, the barons are on the same side as the monarchy and, in the new dispensation, the barons are Russian oligarchs. Director Gareth Machin has a handy company of nine actors, a big scaffolded set by Ellan Parry and a theme of justice for sale.

The first, longer half, comprises Lustgarten's jokey Kingmakers, set in 1225, ten years after the Runnymede signing, with Monty Python barons closing ranks with King John's son and heir in iambic pentameters à la Mike Bartlett in Charles III; and Woodcock's Pink Gin, in which an African president offers his country as a Safari theme park to the highest bidder.

This president, energetically played by Trevor Michael Georges, glugging cocktails and spouting Latin as if possessed by fragments of the MC, sustains interest in a density of voodoo, heavy rainfall and the encroachment of the freedom fighters. The rich, says Sprocket (Mark Meadows), the mediating wheeler dealer in Kingmakers, hold us in contempt, they laugh at us from yachts.

And in Brenton and Wertenbaker after the interval, we see two further stages of this process: the acquisition of the charter by the oligarchs as a tool of barter, and the ultimate ownership ploy, the privatisation of everything – Shakespeare's sold to Putin, the cathedral's gone – in a somewhat woolly scenario (in Wertenbaker's We Sell Right) of post-rebellion meltdown. Only words are left for sale, and words were all the charter amounted to.

There's a very interesting character in the Brenton play, Ellie, a detective inspector (nicely done by Joanna van Kampen), around whom Brenton says in the programme he's building a kind of modern Mystery Cycle; this is his first "contemporary" play for ten years, and there's a tang of excitement – cod Trollope meets up-tempo Midsomer Murders – not least because of vivid performances from Frances Jeater as the befuddled bishop's housekeeper and Michael Mears as the lanky louche canon.

The Magna Carta Plays run at Salisbury Playhouse until 7 November.