Aswollen, humid afternoon - yellowed with light. The back wall and roof of our kitchen and my study above have been demolished. The children return from school to find the naked sky revealed floating, sudden, blue. "Wow!" Chloe grasps the orange horns of a Spacehopper, bounces, "They've taken the lid off our house!"

Builders are bashing trowels on to bricks to dislodge the 100-year-old mortar - a tinking, comforting, nautical sound, masts against rigging. Now and then Jim's mobile rings. "Yup, no, OK," he says. Wheelbarrows spring and slide up wooden planks.

Jacob peers down into the torn-open foundations. "Olden days earth," he observes, hands on hips, grubby legs slightly apart in their grey viscose shorts. "Wonder if they'll find bones."

"Hope not," I say. "But just think, no one's seen this earth since Victorian times - no one's set eyes on it."

"Why is God sitting in a tree in our garden?" asks Raphael, "Can you see him?"

I look. "Is he? No I can't."

"The girls next door did see it - they leaned over and looked in our tree and they did see him."

"Saw him," I correct. "Really?"

"God's invisible," says Jacob lightly.

"It's really true," says Raphael, "I'm not joking. They said he's in our garden." He folds his arms carefully (it's a new gesture for him), "Why can't I see him?"

"Raff, I think it was a joke." I watch the dust rise in the afternoon light - a warm, fluid ocean of dust. I don't think God's anywhere, but I don't say that.

"Why are the builders naked?" (Raphael).

"Why do men have nipples?" (Chloe).

We leave God dangling his legs from the tree and troop back in to watch Wizadora in the cool, blank darkness of our house.

9pm. Hot and dark. Jonathan says, "What's the matter with you these days? You live your whole life as though some disaster's about to happen."

"I don't think it's unreasonable not to want the house to be open at the back all night," I fold my arms - carefully like Raphael.

"It's not open. There's a tarpaulin."

"A tarpaulin. Great, fantastic - why do we ever bother with all those locks?"

"It's a few nights, a week at most. You're ludicrous. What's going to happen? Who do you think is going to come in and get you?"

"I'm not thinking of some mad axe murderer [I am], it's just an invitation to opportunists, that's all."

All totally untrue. If I wake up to find the TV and the video recorder gone, if a teenage burglar gets lucky and plunders our electrical goods while we're out, too bad.

The fear that knocks the air from my lungs, the terror I can't chuck out of my head, is that someone will sneak up the stairs past us in the middle of the night - up to the top of the house where my children sleep, hot-skinned and tender-limbed and open-mouthed. He will creep across the Duplo-strewn floor, past the felt-tipped "Keep Out, Privet" signs and the ticking Bunny Clock, and he will lift a sleeping child from its bed and carry it away into the night.

If I allow my head to contain - even for one second - this uncontainable idea, my gut contracts, a hot wave of panic flushes across my chest, the floor softens, dives, disintegrates beneath my feet. "Get a grip," says Jonathan, "I mean it. You're getting worse. When I first lived with you, it was just sharks in the bath. Now it's everything."

When I moved in with him, he discovered me one morning, naked, leaning over the bathtub, keenly dispersing the bubbles with a bar of soap.

"What the hell are you doing?"

"Getting rid of the bubbles."

"Any particular reason?"

"It just looked a bit, er, sharky," I muttered and gave him a winning smile.

He stared at me.

"Of course there couldn't be a shark in there..."

"That's quite a good starting point."

"But the water's gone all blue, all James Bond-ish. I just can't relax till I can see what's in there."

Now, when this story's wheeled out at dinner parties, I see how ridiculous it was. What innocence, to worry about sharks, when love and parenthood bring so many far worse things. "Every other week you think you've got a terminal disease," he says. "In Greece you wanted to ask that rep whether the ants were 'the biting kind'. And, for the millionth time, people hardly ever steal children. Video recorders, yes, children, no."

"But occasionally..." I protest.

"No one's going to come and get you. Nothing's going to happen to you. You have a nice life. Say it: 'I have a nice life. No one's going to come and get me.' "

"I know I have a nice life. I know that."

"Say it."

"I have a nice life." I look at his nice face and I like it, I love him. My fingers are crossed under the table.

"Say: 'No one's going to come and get me.' "

"No one's going to come and get me." He thinks it's a statement but really it's a wish. Sometimes you have to trick the person you love. Because children disappear, ants bite, people who look OK get ill. Life is fragile, bumpy, hard to hold on to. God isn't in our garden. Heaven is being as happy as I am now and my happiness terrifies me.

I say it. I'll say anything. The words will bounce up into the air, up into the blue, lidless sky. Who knows where they go?