'They only watch those late-night films. There's silence and you're almost asleep, then suddenly there's a crescendo of music and a dreadful shriek'

We're eating supper in my mother-in-law Helen's patio garden by the river at Vauxhall. There is the faintest breeze off the river, but the night is hot. The children, over-excited and hard to settle, are finally asleep, strewn sweatily over the floor in the spare room. Now and then, a party boat goes by - crescendo of disco music, laughter, lights fading. Silvery-black glass on the buildings across the river at Nine Elms; the occasional red bus floats over the bridge.

"Look at those busy lizzies," says Helen. "pounds 3 from Sainsbury's. What do you think, Julie? Though I hate busy lizzies."

"Why did you buy them, then?" asks Jonathan.

"Oh well, I don't know - they were all one colour - I couldn't resist the whiteness of them."

Helen's garden looks especially beautiful this year. Pansies, chrysanthemums, hydrangeas blaze next to sweet william, sugar-pink carnations and gypsophila. Under the striped awning, rosemary, parsley and mint kiss the air, mingling with the sweet, dark breath of the honeysuckle.

"By the way, do you know that you're eating the mint and the parsley from my garden?" she announces as she serves the salad. "Lovely fresh mint. What do you think of that, Jonathan?"

"I really don't know what to make of it," he says, stretching out his legs under the table and pouring more wine.

"It's a triumph," I confirm, my eyes on Ant City, a few feet away on the dark slate of the terrace. Ant City (aka La Cite des Fourmis) was sent to Jacob, six, from Paris by his innovatory godmother. It arrived on his birthday - in winter. The instructions read: Les fourmis sont tres difficile a trouver en hiver. At the time it was definitely hiver. Jacob stood, despite our protests, on the frozen flowerbeds waiting for ants to emerge. Eventually, with bitter regret, the cite was dismantled to await the warmer months.

Now, as luck would have it, his Granny has a plague of ants. They emerge from nowhere, troop over the immaculate kitchen counter and back again. Jacob is enchanted. This afternoon, he captured them one by one in his grubby fingers and installed them in the city. "It's been the most exciting afternoon of my life," he said, black fingers carefully cutting the end off his sesame roll.

"Jacob, why're you cutting that bit off?"

"It's got germs. Raphael touched it."

Raphael stood on the terrace, meanwhile, ants crawling up his Batman t-shirt, over his arms. "Gonna shoot them wid my weapon!" he shouted, poking the air with his bubble gun, his target already halfway up his elbow.

"I hope you're planning on taking Ant City home," says Helen now, as we watch the insects marching up and down through the little plastic tubes, transporting leaves 10 times their size across the sand.

Ants aren't her only problem. In the flat above, there's a mother and son. They listen to their TV - directly above her bedroom - late into the night, keeping her awake.

"The trouble is," she explains, "they only watch those late-night films - you know, there's silence and you're almost asleep, then there's a sudden crescendo of music and a dreadful shriek. I tackled the son about it and he promised to keep it down, but whether they'll manage it remains to be seen."

She gets up, crosses the terrace. "They're in there now," she says, checking. "Sitting there in the dark with the TV on."

The mother - who, incidentally, is way older than my feisty, Lauren Bacall- esque mother-in-law - has also taken to feeding the local pigeons. A group of residents from the flats, including Helen, agreed it could not be allowed, and one man, Mr Koumis, was dispatched to tell her so. The old lady wasn't pleased. "For 35 years I lived in a great big four-storey house in Beckenham," she said, "and fed the pigeons, and I intend to continue to do so here. What's the difference?"

"The difference," the artful Mr Koumis informed her, "is that here you live with other people, that's the difference."

"What did she say to that?" we ask, waiting for a punchline.

Helen shrugs. "As far as I know, she's stopped doing it. And you know the case of the woman in Croydon sent to jail? Well, Mr Koumis has been keeping the cuttings. If we have any more trouble, he intends to post them through her door."

Jonathan laughs. We eat gnocchi and salad off very large, white plates. "Do you know," says Helen. "I've had these plates since before Jonathan was born. I bought them in Cardiff before plain white things were all the rage."

"I always knew you were at the cutting edge," says Jonathan.

"Well, I was. I ordered them specially. They couldn't believe I just wanted white. More than 35 years - just think of that."

There is a sleepy sigh and Jacob is in the doorway, naked and rubbing his eyes. "Jacob! What are you doing? Back to bed."

I steer him back. "What were you talking about?" he asks.

"Nothing," I say, "just admiring Granny's plates."

There's no answer to this. I tuck him in. "Mummy?"

"What?"

"What would you do if Raphael was an ant?"

"An ant? I'd love him just the same."

"But what about when he crawled into bed with you? Wouldn't you crush him?"

"Well, I'd tie a little bell around his neck, like a cowbell - to hear him coming."

He is satisfied with this, the first thing that came into my head. I go back to help to clear the plates which are older than his father.