Jason lifts his eyebrows, then wordlessly switches the button that turns on the emergency response. With blue lights flashing in the grille, and the siren wailing above them, they fly out of the ambulance bay and head up Blackshaw Road towards Wandsworth. They take a right up Burntwood Lane, skirt around Wandsworth Common and pull up outside the Packards’ house in just over five minutes; the first to arrive.

And one look at the smashed window, what’s happening inside, is enough.

Joe throws open the car door and sprints towards the house. He knows the rules, the green light he’s got to enter the property without back-up because there’s an imminent threat to life, but he’s not sure it makes much difference. Seeing someone get whacked with a concrete block is incentive enough. He watches the woman fall to the floor. ‘Get an ambulance!’ he shouts as he hurtles forward.

‘On it!’ Jason’s voice, loud but controlled, filters out from behind him.

Joe reaches the window, covers his hands with his jacket, his face with his hands, and hurdles into the room. He takes in the scene in a split second, then turns to the woman lying unconscious on the floor, blood seeping out of her head. He drops down beside her, checks her airway and pulse. She’s alive at least. He grabs a coat from the sofa and pushes it against the wound. He recognises her, he realises. She works in that dog groomer’s a few doors up from the squat; he was looking through her CCTV footage only a few hours ago. Kat, he remembers her name. But what is she doing here? Is this about Mark? Is she his friend, avenging his attack? But if that’s true, why didn’t she tell Joe that she knew him when he and Jason visited the groomer’s earlier?

‘She broke into my house,’ a small voice whispers in his periphery. ‘Smashed my window.’ Joe shifts his gaze towards its owner. Beth Packard is trembling, but is she scared of the intruder in her home, or just coursing with the adrenalin of hitting Kat with a concrete block? Is she a victim defending her home, her friend, or a dangerous assailant, like Mark claims?

‘Paramedics will be here in seven minutes,’ Jason says, climbing more gingerly through the gap. ‘Hey, are you okay?’

Joe watches Jason turn towards the other woman in the room. It’s the neighbour they met earlier, but she’s lost her mettle now. She looks dreadful, slumped on the sofa, her face pasty white and puffy. Her eyes aren’t focusing, and she’s sweating even though the exposed room is freezing cold.

Jason crouches next to her. ‘I’m DC Worthing,’ he says gently. ‘We met earlier. I think you’re in shock, so I’m just going to lie you down, okay, and put your legs up on the sofa? It will help, I promise.’