It wasn’t difficult: it just took an off-the-cuff remark about sick babies and a comment on the power of celebrity when Stacey called me personally to ask me to forward my solicitor’s details.

I tap my fingers on the steering wheel in time to Glenn Miller. The fundraiser will be the perfect occasion to get to know her better: a professional event, but one that’s emotional, too. It’s hard to keep your guard up when you’re talking about desperately sick children. And I need Stacey to trust me if I’m to be able to help her.

Those livid bruises on her forearms: I can guess whose violent hands put them there, but she has to tell me—

I jump as something slaps against my windscreen.

An empty Styrofoam cup bounces from my car into the road. Someone in the car ahead of me tosses a balled-up fast-food bag from the window. Another plastic cup follows.

As we slow for a red traffic light, I pull into the inside lane and draw level with the vehicle, a gas-guzzling Chelsea tractor. The littering is so blatant I expect to see a harassed mother or French nanny wrestling with a bratty child.

But the environmental vandal is a lone middle-aged woman: glossy, expensively dressed, white. She glances briefly across the car at me, and then shamelessly lobs an empty Perrier bottle out of her window.

A glass Perrier bottle.

It misses a young cyclist by inches, shattering on the road next to him. The boy swerves, too shocked to do more than shout in alarm as he passes the woman’s car. Her response is to give him the middle finger.

Usually I keep my darker angel on a very short tether. We have an accommodation: she holds herself in check and agrees not to set my world on fire, and occasionally, just occasionally, I allow her to have her head.

I never lose my temper. But sometimes I choose to unleash it.

As soon as the traffic lights change, I drop back into the lane behind the Mercedes 4x4. The woman drives with all the consideration I’d expect: cutting in front of other cars, failing to signal, swerving between lanes – I see her texting – and driving straight through a pedestrian crossing without stopping. I lose her briefly near Fulham Broadway when she jumps a red light, but she stops at a patisserie and I soon pick her up again.