The housewives and the hungover prepare to mourn Richard and Judy

"A man's left testicle always hangs lower than his right," said the doctor on ITV's This Morning yesterday, shortly after paying a grateful tribute to the show's presenters, Richard and Judy.

"Is that your left or my left?" asked Richard pensively, glancing downwards at his crotch. "Why do they always ask you which side you dress then?" "Maybe it applies to the willy?" suggested Judy bashfully, manifestly relieved that her husband had resisted the temptation to share his scrotal arrangements with the nation.

Meanwhile, the anxious woman who had called in about her asymmetrical breasts waited patiently for the reassurance that is the show's unfailing prescription for all ailments. It was an entirely characteristic This Morning moment – off-script, mildly amateurish and oddly compelling. When it comes to daytime balls, other broadcasters can't match them.

Until now, perhaps, because at 12.30pm today – petulantly shown the door the day before the Giveaway House contest reaches its climax – Richard and Judy will say their last goodbye to the ITV audience, lured away to Channel 4 by a rumoured £3m contract and the prospect of a very different audience. For 13 years, they have soothed student hangovers and entertained bored housewives, triumphing over a succession of rivals on other channels. It helped their bond with the viewers that Judy graduated with honours from the only form of tertiary education many of them had received – the school of hard knocks. Her appearances on screen with a black eye (the result of an unplanned encounter with a mantelpiece) and struggles with depression, weight and a hysterectomy all proved that the research wasn't just on the presenter's clipboard. Judy had been there too.

The apotheosis of this priceless vulnerability was the moment when she unwittingly bared her ample bosom while collecting a National Television Award, a flash that showed there was more here than just outer shell. Where Anne Diamond – briefly her rival on BBC1 when she teamed up with Nick Owen – was hard and brittle, Judy is a curious mixture of the yielding and the resistant, subject to Richard's blushless revelations about their sex life and her hormonal vagaries (he once confessed on air that he was having a vasectomy because of her irregular ovulation) but also pulling hard on the leash when he was in danger of going too far.

For female viewers, she also represents a realised fantasy – that a figure this mumsy and rounded can secure her very own Mills and Boon pin-up, a man conspicuously her junior in appearance and behaviour. "They're not really mother and son, you know," Mrs Merton told her audience when she interviewed the couple. "They're actually married." It was a joke that pinned down what most people thought – "How on earth did they end up together?"

But that they still were was a large part of the show's appeal. Chemistry had always been claimed for Anne and Nick but it was a grievously synthetic kind, concocted out of office banter and birthday-card sauciness. Richard and Judy, on the other hand, were one of those strange and volatile compounds found only in nature.

Some fans like to talk of their quality as journalists, hinting at forensic talons sheathed beneath the fluff of recipe spots and makeovers. But Tony Blair chose to go on This Morning not because he thought the questions would be more challenging than Newsnight and OJ Simpson knew exactly what he was doing when he made their short-lived evening chat show his first port of call on a post-trial tour of England. On that occasion, Judy was shaking so much she probably had to be gaffer-taped to the studio floor to avoid vibrating off the screen. The truth was that This Morning's scoops were never journalistic but something quite different – unexpected overspills of celebrity candour, such as the moment when Keith Chegwin tearfully confessed to his alcoholism – or contrived nudges at the boundaries of daytime broadcasting such as when they road-tested Viagra on air and celebrated Valentine's day with a gay wedding.

The replacements are already being lined up – Fern Britten and Denise Welsh likely to share Judy's role, John Leslie tipped to succeed Richard – but how they will replicate that curiously embarrassed celebrity is hard to see. Hard, too, to see how Richard and Judy can repeat the trick for a quite different constituency. You can make two predictions for today with absolute confidence: there will be tears before bedtime (Judy is famously leaky at emotional cruxes) and the next show won't last anything like as long.