So you're sitting in the pub and your nice-but-naive friend says: "Hey, I heard the most amazing story the other day. There were these twins, right, a boy and a girl, who were separated at birth and adopted by different families. And, like, years later, by an amazing coincidence, they meet. And fall madly in love, and get married. Straight up! Then, obviously, they find out they're actually brother and sister. And it all has to be annulled, and they're just devastated. It's the ultimate nightmare. Can you imagine?"

Assuming your brain is still functioning like the well-oiled piece of precision engineering it is, your response would, I trust, be: "That's a wind-up if ever I heard one. Think about it for a minute - you mean these two meet by accident, discover not only that they were both adopted but were born on exactly the same day in exactly the same town, and still never pause to wonder whether they might be related? Pull the other one. What did it say on their birth certificates?"

Last week it was reported that this unlikely scenario had actually taken place - and news outlets the world over went bananas. Biologists and psychotherapists came forward to confirm the "genetic inevitability" of the pair's attraction and lamented the "cruelty of fate" and "trauma of forced separation".

The tabloids screamed: "Are YOU one of the twins - or do you know them? Get in touch NOW on 020 ..."

Here's the thing: it all came from a single remark more than a month ago by the vehemently anti-abortion Roman Catholic peer and father of four, Lord Alton, in favour of all children having the right to know the identity of their biological parents.

He had heard about this particular case, he said, from the judge who handled the annulment. Or perhaps (he later admitted) a judge who was "familiar with the case". Britain's top family judge, Sir Mark Potter, has never heard of the story. And, as the excellent Heresy Corner blog notes, the whole thing is statistically improbable, procedurally implausible (for 40 years, adoption practice has been to keep twins together) and based on the equivalent of a friend in the pub saying, "Hey, I heard the most amazing story the other day."

Still, stranger things have happened.