David Hare's classic conversational drama considers our windows on the world. Its title invokes a dying woman, Tom's wife Alice, lying in bed, staring up at the sky. At the end of her life, she stopped taking in the world around her and turned her eyes upwards instead.

Kyra's council flat looks out over Kensal Rise. Privileged and principled, she gazes out of her sitting room window as snow starts to fall. Her front door is frosted, obscuring the world beyond. When she commutes to work, to a school all the way over in East Ham, she stares down at the city from the top deck as if taking an open-top bus tour. She imbibes London life, all its hustling, bustling, struggling citizens, but she herself floats above it.

Wealthy restaurateur Tom, meanwhile, her fifty-something former lover, lives in a "warm bubble of money and good taste" in SW19. Just off Wimbledon Common. All his friends are much like him. His only contact with another class is his driver. Who said echo chambers and bleeding hearts were unique to social media?

Skylight brings the two of them back together. A year after Alice's death, three after their affair ended abruptly – Kyra dashing off, Cinderella-stylee, the moment it came to light – Tom swishes into her flat bearing whisky. While their encounter is warmed up by old flames, it's mostly designed to set up a simmering ideological battle: his self-interested Thatcherism, living life to the full, against her overbearing empathy, the sort that sacrifices itself on the altar of social injustice.

She's a brilliant character Kyra, another of Hare's complex women. She embodies all-or-nothing saintliness. Casting off any privilege, she lives a puritan, joyless existence in her freezing cold flat, surviving on spag bol and classic literature. Her asceticism is, you realise, an aesthetic one and it's enough to make you cheer Tom's boozy, boyish indulgence – especially if, by employing people as he once did Kyra, he does more practical good than her. She seems wasted in teaching: a brilliant young woman indulging herself by 'making a difference.' Might she make more elsewhere?

Kyra's flat floats on a bed of breeze blocks in James Perkins' design, a concrete city seen from above. Miniature lights make each a home, and you think of all the little people below. Kyra maroons herself away on this island. Her council flat's a castle, and she peers over the balcony to buzz guests in like London's own Rapunzel, letting down her hair. Next to sunken-in sofas and formica surfaces, her grand wooden bed whiffs of Sleeping Beauty.

All very neat, but Tamara Harvey's production is too temperate by half. There's too little at stake – no tension between them, yearning or pain – so their encounter tips too far into nostalgia; not present-tense, but past. Without emotions intruding, their ideological tussle flattens into ration debate. Jeany Spark cradles herself against the cold as Kyra, playing piety with soft-focus saintliness, but Jay Villiers can't muster the heat to get the play's blood pumping. His Tom's in disarray, softness having set in, but he's neither a suave silver fox, not a Grade-A shit. Against Kyra's stoicism, it's Tom's job to pull our sympathies this way and that. Why would she abandon principles for him, and why should we care if she did? Neither Villers nor Harvey ever solves that.

Skylight runs at Clwyd Theatr Cymru until 4 March.