YOU will struggle to believe this, but there are many differences between myself and Keira Knightley.

Unaccountably, she’s never asked me to trade places, and if she ever offered now, I would have to think twice. Not because I couldn’t face the talent, the beauty and the slinky-minx wardrobe, but because Keira and her mother not only acknowledge that sex exists: they actually talk about it.

To one another.

If recent reports are to believed, Keira says her mum, Sharman MacDonald, is a “chilled-out hippy” who would often come into her room to share a ciggie as the two of them chewed the fat over matters far more intimate than the colour of Keira’s curtains.

There’s worse. Sharman’s a writer whose recent work has involved devising sex scenes for her daughter to perform in her latest film, The Edge Of Love, about the poet Dylan Thomas.

My insides curdle at the very thought of such chummy-mummy stuff.

If my mother ever came up to my bedroom, it would be to force the door ajar and order me to scrape last year’s laundry off the floor.

And the idea of Mum ever inviting me to roll up a tab for her is hysterical. The last time she ever slapped me was when she found the ten-pack of Player’s Number Six that, aged 14, I’d bought from a machine outside the newsagents and smoked in the sitting room while she and my Dad were out.

I had made the schoolgirl error of imagining the smell would somehow vanish by the time they got home from the party.

After the age of 25 she thought I had quit the evil weed and I never admitted to her that I’d taken it up again. Even in my early 40s I tried to keep it from her. If you’re reading this, Mum, I did eventually stop smoking – four years ago.

One of the most embarrassing moments of my young life was when, for an am-dram production at the age of 16, I was cast in the role of The Other Woman, done to death by an errant husband. The husband was played by my father, and the experience of playing opposite him was grim in the extreme.

This was West Yorkshire in the Seventies, so the play was innocent of any physical stuff, but the very notion of playing such a role put me right off my O-levels. That’s my excuse, anyway.

Yes, the quease-making subject of S.E.X. should be strictly off-limits between the generations. By all means make sure everyone knows what goes where and what the consequences can be, but thereafter, for heaven’s sake let it lie.

I have come to accept that my parents must have worked out how things work back in the 60s, because I am ageing in exactly the same way that they did before me. My childhood conviction that they abducted me from my true destiny as a diamond heiress has had to bite the dust.

I also have to accept that whatever happened when I came along was not a freak accident, because I have a brother and sister who look a lot like me and not unlike my Mum and Dad, too.

But knowing all this and discussing it are different. I can no more imagine opening my heart to my parents about such matters than I could bungee-jump off York Minster.

Mums are mums and dads are dads – they love you, you love them, but they are not your bessie mates.