"Down there," he points, steps quickly back behind the curtain. It's them - we call them The Couple. They've been here every day since last Tuesday - maybe longer, but that's when Jonathan first clocked them. Their two cars draw up outside our house. In the front car there's a man - 45- ish, bubble-permed hair like a footballer, toffee-coloured suede coat with sheepskin lining.
"That's called a car coat," I muse aloud. "Isn't that what you call it?"
Jonathan pulls a chair up to the window, sits down just out of sight. "He just looks like a garage owner."
In the second car, usually arriving a few minutes later, there's a young woman and a baby, maybe six months. The baby's strapped into a carry-seat, the young woman is blonde and round-cheeked and wears a pink dufflecoat.
"Is the baby hers? Or is she just the nanny?"
"She's very young, what - 19 or 20?"
"Old enough to have a baby."
"More to the point, is the baby his?"
"Why come in separate cars, then?"
It's always the same. The woman gets out of her car, picks up the baby and goes and sits with it on her lap in the back seat of the man's car. He stays in front. "That's not very safe, if they're intending to drive off," I mutter. "That child's not strapped in."
"God, you're boring. Let's see what happens."
What happens - with him still in the front and her in the back - is a certain amount of canoodling.
"Canoodling? Christ, you really are an unsophisticated woman."
"What would you call it then?"
"Snogging, Frenching. Making tongue-sandwiches."
"But they're not actually doing any of that, are they?" I protest. "Mouths aren't involved. He can't reach round." There's no kissing, just leg stroking, earnest talking, finger-twining, still with the baby on her lap. "Why doesn't she go and sit in the front?" frowns Jonathan, really bothered. "Why make it so bloody difficult?"
After five minutes of this, the couple drives off in the man's car, leaving the girl's car parked in our street. "You see! Driving off and that baby is not strapped in. Unbelievable."
"What are they doing? Where are they going? A motel, or his place? And why waste time snogging and why in our road?"
"Illicit love affairs are all about sitting in cars in leafy, residential roads," I say, putting him straight. "That's what you do. That's where you go, to be safe."
"And you should know," says Jonathan - referring to the fact that, long ago, years before I met him, I fell seriously in love with a married man. I shared a flat with three accountants who were always in, watching TV and anxiously washing their socks. There was never anywhere to go.
When we could no longer brood on our mutual pain in pubs or on wet pavements, we sat in his car, parked outside other people's houses in places we'd never heard of, where no one we knew would ever go.
I remember the coming and going, all those (enviably open) lives going on around us, the slamming of car doors, crying of alien kids, yapping of unseen dogs. I remember being so in love that waiting, watching the side of his perfect face seemed enough. Most of all, I remember the unchanging banality of all that sitting - the crying, the kissing, the going nowhere. I was 22 and he was old enough to know better.
"Don't bring my past into this," I tell Jonathan.
"You brought it in. Actually."
I have an idea. "Hey, maybe the baby is his baby, but she has to pretend it's her husband's and so these are their few precious moments together as a family."
"But then why stick with the husband? Why not just leave him and get on with it?"
"Well, maybe it's not that easy. Maybe the husband's stinking rich. Or crippled."
"Or blackmailing her."
I ignore this wild, unlikely pitch into criminality. "She could be a nanny, That dufflecoat does not say `mother' to me."
"Jenny has a dufflecoat." (Our friend Jenny who has twins).
"But not pink. It's a fluffy colour" - I recognise it as the sticky, sock-it-to-me pastel of emotional virginity. "A sex kitten colour."
"That's her written off then."
"Jonathan, why are you getting at me?"
"Sorry, I'm lost."
"You've gone all hard. Sarcastic."
"I'm always hard. I'm the Hard Man of Clapham."
"Why do you do this? You've been horrid ever since they drove off." I realise we are still standing at the window. The objects of our curiosity are long ensconced in some motel, while we stand framed in the window, arguing.
"Hang on, this is the normal me. You've suddenly gone all sensitive - tormenting yourself with your grand amour fou."
"That's it, isn't it? You can't stand me talking about past love affairs."
"You barely ever talk about anything else."
"You think you don't mind," (I'm triumphant). "But deep down it really annoys you."
He stands, puts the chair back. "Do you want some lunch?" "What're you having?" "Pasta." "We haven't got any parmesan." "Just on its own then." "If you cook it." "OK, OK."
We move away from the window. White, empty space. Show over.