Somehow, Miniver is on that list of emergency numbers. By luck or by design, Marnie's place is actually near the Tower -- near enough that Miniver is within two blocks of Cass when he's hit, having been sent out by Marnie to take a walk before he wears a hole in the floor with all his fidgeting and pacing.
This is a part of London he knows well; he also loves the history of the place, and the aesthetics of it. He's been here many times. Less than five minutes pass before Miniver comes running to the scene, shoves his way through any surrounding gawkers, and skids to a flailing halt over the ice to Cass's side.
In a panic, he does one of the few things he knows how to do and feels for a pulse. It's there, but that doesn't erase the sight of Cass's twisted body.
"Did someone call an ambulance?" he demands of whatever crowd has gathered. "Fuck, is NO ONE here a doctor?? HEY! You!" He focuses on a person with a phone in their hand. "Call [number] and ask for Sable. Tell anyone who picks up it's an emergency." And in the seconds it takes to dial and connect, Miniver is emptying his pockets. He grabs the ohone when it's offered to him and babbles at his daughter about what he sees, using his handkerchief to tend to the bleeding on Cass's face as well as he can until the ambulance arrives, as she instructs.