We follow them from a distance. Now, this is exciting. I feel like I’m a character in a film. A private eye. The man is tall and slim. He’s wearing board shorts and a T-shirt and he’s got a navy baseball cap on the wrong way round. His arm is loose around Catherine’s waist and she’s leaning up against him as they walk. Her shorts are cut so high you can actually see her bum cheeks. Every so often the man’s hand slips away from her waist and down toward the back of her shorts. One time he does this, he grabs a handful of her right bum cheek and gives it a squeeze. Catherine doesn’t even flinch. She just lets him.

We’ve crossed over the bridge now and are heading for the subway that goes under the dual carriageway.

“They’re definitely going to do it,” Alice says, her eyes glued to her sister’s bottom.

We follow them down into the underpass, waiting till they’re almost at the end of the tunnel before we set off after them. When we see which exit they’ve taken we speed up.

Out in the sunlight again, it isn’t immediately clear where they’ve gone, and for a few seconds we think we’ve lost them. But then I spot them in the car park, climbing into a blue car.

“Shit,” Alice says. “That’s the end of that, then. They’re just going to drive off, aren’t they?”

Except the car doesn’t go anywhere. We climb over the low wall that separates the car park from the pavement and creep as close as we dare, keeping low so that we’re hidden by the other parked cars.

“Hang on,” I say. “Where’s she gone? I can only see him in there now. She must have got out.”

Alice peeps round the white van we’re now hiding behind. “No, she’s still in there.”

“But why can’t we—”

Alice looks at me and inserts her index finger into her mouth, starts sucking it and pushing it in and out.

I clap my hands to my mouth and collapse in a fit of giggles.

“Look at his face, Lizzie. Look at his face!”

I squint to get a better view. It’s hard to see much from this distance but I can just make out that he’s extended his head back onto the headrest so that his chin is sticking up in the air.

Alice retrieves the packet of Hard Gums from her pocket. There are only two left now so we have one each.

We watch. We wait. We chew.

“How long do you think it takes to give someone a blow job?” I ask, my mouth still full of Hard Gum.

Alice smirks. “Depends how good you are. They like it more if you swallow.”