You can’t accuse Glenda Jackson of sidling back onto the boards. At the age of eighty, after a quarter of century away from theatre as MP for Hampstead and Kilburn, here she is making her first return to the stage in a gender-blind assault on the most daunting role for senior actors in the Shakespearian repertoire. In terms of post-parliamentary chutzpah, is there even a remote equivalent to this? Only a light-hearted one springs to mind. Gyles Brandreth was the MP for Chester for five years and went on to play Lady Bracknell (rather well) in a musical version of Wilde’s The Importance of Being Earnest. But his break from showbiz was comparatively brief and he doesn’t have quite as distinguished a back catalogue as the two-time Oscar winner who has been electrifying audiences since Peter Brook recruited her for his ground-breaking RSC Theatre of Cruelty season in the mid-1960s.

Watching Deborah Warner's modern-dress production at the Old Vic, you simply can’t believe that Jackson has not been on stage for 25 years. That metal-tipped whiplash of a voice is undimmed in its power to inflict devastating, incredulous scorn as we hear from the moment that Morfydd Clark's excellent, refreshingly honest and mettlesome Cordelia refuses to play the game of lip-service in the opening love contest. A terrible fierceness of spirit animates the slight, rather frail figure this Lear cuts. Made aware for the first time of the enormities of social injustice by his outcast state during the storm, Jackson’s king delivers the lines “O, I have ta’en too little care of this” is if a red-hot needle of regret had been inserted into his brain. She’s hair-raisingly vehement as she gives vent to Lear’s misogynist rants and finds quiet touches that piercingly bring home his anguish. This Lear is dragged in on a silver cloth cradling the corpse of Cordelia and the tripping rhythm Jackson gives to “Never, never, never, never, never” as if it were a line in a song she was improvising to comfort the dead girl, breaks your heart. Hers is a portrayal of the hero that passionately vindicates the principle that, when it comes to casting, it’s the insights and instincts of the performer and not his or her gender that’s of paramount importance.

Warner's production is uneven and unmissable – offering, at its best, a powerfully imaginative vision of the stark, pitiless incoherence of the universe in King Lear. The storm and heath are thrillingly communicated. A curtain like a gigantic black bin-bag flops down, turbulently billowing and bulging over the minimalist white stage. It’s bin-bag plastic out of which Harry Melling’s Edgar fashions a crude, make-shift loin-cloth during his brilliantly frantic and unnerving impersonation of “Poor Tom”, the bedlam beggar. But some of the casting is very poor and the deliberate distancing devices – act and scene divisions flashed up; actors seen mixing with technicians; the opening love test conducted almost like a rehearsal in its spareness – paradoxically seem to seal the production off in its own theatrical reality rather than invite us to think harder about the play’s extraordinary resonances with our current world of an appalling refugee crisis, widening inequality and worsening inter-generational conflict.

There are going to be background talks about all of these connections but we want to feel them more in the texture of the production. There’s another paradox: a Lear with a female actor cast as hero seems to have gone back to the pre-Peter Brook conception of Regan and Goneril as somewhat camp villainesses instead of as the victims of their father’s unreasonable demands. Are they the product of different kinds of paternally induced trauma. You aren’t kept sufficiently guessing by Jane Horrocks’s Regan stalking about on her killer heels and Celia Imrie’s pursed, suburban Goneril, who, after poisoning her sister, dons the marigolds to scrub away the vomit.

The characterisations are so hit and miss. Rhys Ifans is a lovely warm and robust Fool, wandering about in a bedraggled Superman outfit and sticking two eggshell halves into his eye sockets as a larky but pointed parody of his master’s moral blindness. Simon Manyonda, by contrast, gives a dismayingly external account of the malcontent bastard Edmund who is here largely reduced to a cocky show-off who at one point treats us to the sight of his bare behind while he has a swift and noisy wank.