Have you ever tried to make a castle with a pack of cards? You start very carefully, and - if you know the trick of licking the edges of the cards, one by one, to make them stick together - you can probably raise your masterpiece to a towering two or three floor building. Four, with a bit of practice.

As you keep adding the cards, though, you may realise that the structure is rather wobbly and won't withstand the addition of another floor, or even another two cards precariously leaning one against the other, at the top. Have you ever felt the impulse to destroy the whole thing with a swift blow, taking a secret pleasure in seeing it collapse flat on to the table, cards scattered everywhere?

I spent today with my Beautiful Stepdaughter. She commented, all of a sudden, that her father and I are 'complete strangers, living under the same roof but leading parallel lives.'

She is sixteen. I was rather taken aback and not a little ashamed; I clearly have failed in hiding the wobbly castle of cards from her.

'Why won't you speak to my mother?' she asked.

'Your mother?' The notion that I should speak about Mr Husband to Mrs Ex-Wife is alien to me, although, over the years, she and I have had a jolly pleasant relationship, mainly made of smiles and the occasional cup of tea.

'Yes. She will understand you.'

'It's funny that you should mention your mother', I answer, carefully. Then I add: 'when we were arguing last night, your father sneered at one of my comments about us and told me I sounded just like your mother, before they split up.'

Beautiful Stepdaughter shakes her head. The boys are two dots in the distance, over the bridges across the lake, blissfully unaware and running towards the sunset in Regent's Park. We are walking at our own pace, occasionally calling out when we think that they are too close to the edge of the lake - they like to push the ducks into the water.

'That's because he is in the same place as before, but doesn't realise it.'

Mrs Ex-Wife met another man and ran into the sunset with him. It did not last, but that's another story.

I look at Beautiful Stepdaughter and am surprised at how tenderly I love her. Even now, after so many years, I cannot believe how beautiful she is, and perceptive.

I don't need to say anything else.

'You can't live like this. Speak to my mother.' She could be my daughter, but is infinitely wiser than I.

The tam-tam of the 'Jungle Girlfriend Network' seems to include even Mr Husbands' ex-wives. I am not sure I will take up the suggestion, but I look at my own castle of cards, and deal the fatal blow. Silently; he won't know for a while. But I feel better for it.