No. Stop it. I need to bond with her. I’ll talk about gardening.

‘Come out and see!’ I say, opening the back door and ushering her out on to the tiny patio. ‘I mean, we don’t do much with it … Do you have a boyfriend?’

Oh God. That just popped out before I could stop it. Did that sound unnatural?

No. It’s fine. It’s a normal question. It’s what you do when you meet people. You ask them about themselves.

‘No.’ Mary’s face twists into a rueful expression and she wanders over to look at our sole tree, a silver birch. ‘Not for a while.’

‘Ah.’ I try to sound understanding, like a member of the sisterhood, not like the suspicious wife who’s mentally logging no boyfriend.

‘Men can let you down so badly,’ Mary continues in her melodious voice. ‘Or maybe it’s just the men I’ve come across. They seem to have an extra capacity for deceit. This is lovely,’ she adds, stroking the tree.

She has picked the one thing in our garden you could describe as lovely.

‘And yarrow!’ she exclaims, reaching for some nondescript plant I’ve never even noticed. ‘Gorgeous. So healing. Do you ever use it in your bath?’

‘Er … no,’ I admit. Use that scraggy plant in my bath?

‘Never let anyone tell you that it’s a weed. You can make a wonderful tincture with the flowers. It helps with sleep … fevers … everything.’ She looks up, her eyes shining, and I stare back, slightly mesmerized. ‘It’s one of my passions, natural healing. And energy healing.’

‘Energy healing?’

‘Using the body’s own energy to rebalance.’ Mary gives me her beatific smile again. ‘I’m only a beginner, but I believe passionately in the mind–body connection. In the flow.’ She gestures down her body in one beautiful movement.

‘Here you are!’ Dan’s voice interrupts us and we both turn to see him stepping out of the back door. ‘What are you two gossiping about?’

He sounds self-conscious, I instantly register. Too hearty.

‘Sylvie was asking me about my love life,’ says Mary with that same rueful expression, and I see Dan’s gaze dart to me sharply.

Great. So now it looks like I’ve led Mary outside, away from the group, to demand if she’s single.

Which is totally not what I did.

I mean it’s not what I meant to do. It just came up.

‘I wasn’t!’ I say a little shrilly. ‘I mean … who cares about that?’ I attempt a laugh, which doesn’t quite come off. ‘Anyway, tell Dan about your natural healing, Mary! It sounds amazing!’

OK. So I’m being a bit Machiavellian here. If I had to vote for Person Least Into Alternative Medicine, it would be Dan, by a million miles. His view on medicine is basically: take paracetamol and see your GP if you really must. He doesn’t take vitamins, he doesn’t meditate and he thinks homeopathy is a massive con-trick.

So what I’m hoping as we sit down to dinner is that as Mary talks about ‘mind–body flow’ and ‘clearing energy blockages’, Dan will adopt his usual cynical stance and the two will end up arguing. Or at least disagreeing. (Dream scenario: Mary stomping out of the house, shouting, ‘How dare you say reiki is all a load of bollocks!’)

But it doesn’t happen like that. As Dan doles out the lamb, Mary tells us about her healing in such an intelligent, compelling way that we all listen, riveted. She sounds like a Shakespearean actress. She even looks like one. I start to think maybe there is something in healing after all, and even Dan seems quite open-minded. Then she moves on to yoga and teaches us all a shoulder stretch at the table. And then she tells funny stories about going on a herbalism course and making some kind of beech-leaf liqueur and everyone getting totally drunk.

She’s not just angelic, she’s sassy. She exudes positive energy. Everyone is charmed. I’m charmed. I want her to be my friend.

As the evening progresses, I find myself relaxing. My fears seem to float away. There’s no special vibe between her and Dan that I can make out. Dan has relaxed too, and he seems just as interested in catching up with Jeremy and Adrian as he does in Mary. By the time we’re on to the Green & Black’s chocolates, I’m thinking: We must do this again, and: What nice new friends, and: I’ll ask Mary where she got those grey pumps.

I’m just pouring out fresh mint tea when I realize that a shrill voice is calling for me: ‘Mummy! Mummy!’ and excuse myself. I find Anna standing on the stairs, clutching the banisters, her face wet with tears, telling me, ‘It was coming after me, it was coming, it was coming.’

Poor Anna. She always takes ages to calm after a nightmare, and so I settle down to a good twenty minutes of sitting on her bed, soothing, patting, singing and talking in a low voice. She seems to drift off, then opens her eyes in panic and searches for me … then drifts off again … then opens her eyes again … and I just sit there, patiently, waiting. And at last she’s truly asleep, her breaths coming deeply, her fingers still clutching the edge of her duvet.

I feel tempted to climb in with her. I’m suddenly quite shattered. But after all, we still have guests and those Green & Black’s chocolates won’t hand themselves round. So at last I get to my feet and head out of her room … and freeze dead. From where I’m standing on the landing, I have a view into the mirror in the hall, and in that mirror I can see the sitting room reflected.

And in the sitting room are Dan and Mary. Just them.

They must have no idea I can see them; that anyone can see them. They’re alone and standing close together. Mary’s listening to Dan, her head tilted with an intent, understanding expression. He’s talking softly to her – so softly, I can’t hear his actual words. I can pick up on the vibe between them, though. It’s a vibe of closeness. Of familiarity. Of everything I was afraid of.

For a few moments I’m motionless, my thoughts lurching this way and that. I want to confront them. No, I can’t face confronting them. I might be wrong. Wrong about what, anyway? What do I imagine is happening? Might they not just be two old friends sharing a moment?

But why hide away from everyone?

A burst of male laughter from the kitchen brings me to, and automatically I start walking forward. I can’t stay upstairs forever. As I head downstairs, the stairs creak, and at once Dan appears at the sitting-room door.

‘Sylvie!’ he exclaims, too loudly. ‘I was just showing Mary …’ He trails off as though unable to think of a convincing story. Then Mary appears in the doorway – and the look she gives me makes me feel chilled. It’s unmistakable. It’s a look of pity.

For a moment our eyes are locked and I swallow, my throat tight, unable to speak.

‘Actually, I must be going,’ says Mary in that soft voice.

‘Already?’ says Dan, but he doesn’t sound too sorry, and as we head back into the kitchen, the other two are also standing up, talking about tubes and Ubers and thanking us for a wonderful time.

This evening has got away from me. I want it to slow down. Press pause. I need to gather my thoughts. But before I have a chance to, we’re in the hall, finding everyone’s coats and exchanging kisses. Mary won’t look me in the eye. I’m desperate to pull her aside and ask, ‘What were you talking to Dan about just now?’ And ‘Why did you two go off like that?’ But I’m not brave enough.

Am I?

‘Mummy! I woked up!’

Tessa’s shrill voice interrupts my thoughts and my heart sinks.

‘Tessa! Not you too!’

I hurry upstairs instinctively, scooping her back before she decides to join in the party. Children being out of bed is like the five-second rule – you have to be swift. I bundle her back into bed and sit there until she’s closed her eyes, listening to all the final goodbyes below in the hall and the front door closing. When Tessa is gently snoring, I creep back out on to the landing. And I’m about to head downstairs, when something stops me. Something hard and splintery and suspicious. Instead, I move silently into the bathroom which overlooks the front of the house and peep out. Dan and Mary are on the pavement, talking, just the two of them.

How did I know they would be there?

I just knew.

There’s a horrible squeezing feeling in my chest as I crouch down by the window and silently open it a crack. Mary has wrapped herself up in a pashmina and her face under the streetlight is wreathed with concern.

I lean my head against the windowsill, trying desperately to pick up scraps of conversation.

‘Now you understand,’ Dan is saying in a low voice. ‘I just feel … pinned in a corner.’

My throat tightens in shock. Pinned in a corner? He feels pinned in a corner?

‘Yes. I get it,’ Mary is saying. ‘I do. I just …’

Their voices descend lower, and I can only pick out the odd word. ‘… talk …’

‘… find out …’

‘… she won’t …’

‘… be careful …’

My heart is thudding as I peep out of the window again to see that Mary is clasping Dan in a hug. A tight hug. A passionate hug.

I sink back on my heels. I feel faint. Dark shapes are scudding across my brain. Am I the ultimate, trusting fool? Were the pair of them playing me all evening? I’m rerunning Mary’s friendly, charming air. Her soft voice. The hand she kept putting on my arm. Was it all an act? ‘Men … seem to have an extra capacity for deceit,’ she said – and now I remember the look she gave me. Was that a hint? A warning?

I hear the front door closing and hastily come out of the bathroom to see Dan in the hall, staring up at me. There are shadows on his face and I can’t read his expression and all I can think is: He feels pinned in a corner.

‘You go to bed,’ he says. ‘I’ll just stack a few plates. We can do the rest tomorrow.’