Thanks partly to the Glasgow Citizens, ironically enough, we are far more familiar down south with Schiller's great fictional encounter between Queen Elizabeth of England and Mary Queen of Scots, although that play presents a rather stately version of their conflict.
Lochhead's text - raw, rough and sometimes as knottily and Scottishly idiomatic as any flight of Robert Burns - presents interwoven portraits of younger women (Mary was an 18 year-old widow when she returned from France, Elizabeth 27) who in turn alternate - by flicking their dresses inside out - as their own respective servants.
Sarah Thom as a surprisingly fierce and red-haired Elizabeth, and Nora Wardell as a more fluently emotional and dignified Mary, manage these exchanges, and these changes, magnificently. And their respective love lives are pointedly contrasted in their dealings with Sean Hart's devoted Darnley and (the unseen) Earl of Leicester.
But Lochhead is presenting a sort of historical cabaret, supervised by Shelley Lang's vituperative, dangerous crow, the national bird, La Corbie (a wonderful role first played by Myra McFadyen and later by the shape-shifting Lisa Kerr), and for all its vim and vigour, these seismic shifts of double dealing and rivalry are smudged and lost in a tangle of references which demand a close knowledge of history.
This is the moment, after all, as Joyce MacMillan once memorably expressed it, when Scotland rejects its own beautiful Catholic queen and turns to the harsh revolutionary Protestantism of John Knox and his followers, causing untold damage to a national culture in the process. There's not any real sense of that at the King's Head.
What we do get, with the audience seated on three sides, reminding us that this pub back room was once a boxing venue, is an endlessly inventive, somewhat exhausting, staging of the power shifts and spats in the endgame, with Prentis Hancock's unexpectedly tentative John Knox sniping on the side lines, blowing the first trumpet, he says, against this monstrous regiment of women.
There's only one result, as chanted in the playground song of the title and absorbed into an ingenious narration of the execution. Wardell and Thom are full-bloodedly engaged in this contest; to such an extent that, while there's a victory of sorts for one of them as a queen, we must declare a points draw for the actors.