Revived from a 2015 run in Stratford-upon-Avon starring Natascha McElhone, Helen Edmundson's history play transfers to the West End with another glamorous leading lady in the form of Romola Garai. She seems perfect casting for Sarah Churchill, Duchess of Marlborough, the ardent, appealing yet hugely arrogant and manipulative member of Queen Anne's court – her closest confidant who slowly loses her grip as the monarch finds her own.

Garai sashays round in a series of sumptuous scarlet dresses as the Whiggish Sarah, with her usual mix of steely determination and irresistible warmth, lit by that sun-beam smile. She's matched by a quavering portrait from Emma Cunniffe of a sickly queen who's lost 17 children; she's a baby-voiced, hang-dog creature who leaves us wondering just how knowing and ruthless – or noble but vulnerable – the Queen might really have been. And although there are real letters of adoration from Anne to Sarah, Edmundson allows their relationships to remain ambiguous. Was Anne's neediness a yen for more than friendship? Was Sarah merely politicking in her affection?

Stir into the mix a seemingly morally upstanding maid named Abigail Hill (an astringent Beth Park) who nonetheless inveigles herself into the Queen's affections in service of the Tories, and the play forms a study of female ambition and machinations in the service of the future of the country, where the personal is always political. Friendships may be a means to an end, but Queen Anne also insists on the particularly potent charge a close female friendship can carry – and how that can turn sour.

But I wished it would dig further into the personal – give us a greater sense of their past, their relationship – and leave off the politics a bit. There's an awful lot of little-known history stuffed into Queen Anne, which begins in 1702; layers and layers of Tory and Whig rivalry, Catholic and Protestant struggles, wars between Britain and its European allies against France and Spain, all puffing out like a petticoat – and hampering the play's movement. It can be slow, stately. Edmundson mostly avoids it feeling too much like a history lesson – but then you might feel like you needed one beforehand to really follow it all.

Although enlivened with grotesque satiric performances written by pamphleteers such as Jonathan Swift and Daniel Defoe, under Natalie Abrahami's direction Queen Anne is too often stuffy and stiff rather than pulse-quickeningly theatrical. It fulfils old-fashioned expectations of ‘the sort of thing' the RSC does – although really, it could transpose wholesale to the screen as a lavish costume drama. There is also some lazily ye olde dialogue: Hill's claim "I am just a simple country girl" isn't delivered knowingly enough, for instance, to save it from cliché.

Still, Edmundson has found three complicated female characters to bring to life, and performances from Garai, Cunniffe and Park keep us guessing - and keep Queen Anne an intriguing proposition.

Queen Anne runs at the Theatre Royal Haymarket until 30 September.