One evening in 1961 she made passing reference to the teenage idol of the moment, "that chubby Cliff Richard". The remark stung. The 12-and- a-half stone 21-year-old pop singer decided to go on a diet and get fit. The path was set for doing without lunch for 33 years (he has started to eat at midday again recently) and the daily tennis which have promoted the look of eternal youth as well as a deal of sneering (he once made the mistake of admitting that he preferred tennis to sex).
The conversion to lunching is not, apparently, a sign of weakening with the years. Cliff has never done that. It is due to his decision to put on a stone (from 11 to 12) for the eponymous lead role in Heathcliff, a musical version of Emily Bronte's Wuthering Heights, which this week set a UK theatre record for advance sales when pounds 2.3m worth of tickets sold on first day of booking.
The surprising thing is that anyone should be surprised by the success. Sir Cliff Richard is without doubt, as one pop pundit had it, "the most successful British chart act of all time". He has sold 45 million singles over a 30-year period and is the only UK artist to have had hits - more than 100 of them - in every decade from the Fifties to the Nineties. Yet those who are not fans continue to puzzle over his enduring attraction.
The standard response is a faint derision. Those who camped out for days for tickets on the fourth floor of a car park at the National Indoor Arena in Birmingham were described as "ladies of a certain age" (45 to 65).
Recent reports that Cliff is the man most women would want as their partner on a fantasy holiday (Mel Gibson was second) are qualified by the fact that the survey was conducted by Hoseasons. Newspaper profiles often carry the jibe that he is the same age as the rail union leader Jimmy Knapp. Urban myths about this Mary Poppins of the pop world include the malicious, and wholly erroneous, suggestion that he is the bearer of a colostomy bag. What is the cause of this sniggering and leering?
It might be his middle-of-the-road musical style. It might be his relentlessly Man at C&A fashion sense, which has always changed with the times but never departed from that basic template, albeit always choosing clothes designed for a man younger than his years; he is 56.
It might be his unashamed avowal of the very direct evangelical style of Christianity. When the Beatles were sitting at the feet of the Maharishi Mahesh Yogi in India, Cliff (who was actually born in Lucknow, spent the first eight years of his life in colonial India and endured racist taunts for his sunburnt skin when he arrived at school in England) was being called up to the stage at Earls Court by the US evangelist Billy Graham.
It might even be his personal asceticism: in an era of material affluence Cliff revels in personal frugality - despite reportedly being worth pounds 25m he delights in underspending on the meagre pocket money he awards himself and gives up considerable time to working for Third World, medical and children's charities.
In fact the core reason is none of these. Sex is at the heart of both his success and the smirking. Go to his concerts and you will see that sex is what the public Cliff Richard sells, in a peculiarly antiseptic and choreographed form, to his middle-aged female fans. Yet sex is something in which the private man seems to have virtually no interest. The paradox is what titivates.
The pop idol's primary duty has always been, as the writer Philip Norman once memorably said, to represent sin it its most enviable forms. Cliff Richard offers almost the opposite. No drugs, no foul language, no guitar- smashing or room-wrecking, no large-scale self-indulgence and no sex.
It was not always so. In 1958 his television debut was greeted by the New Musical Express with tirades against TV depravity and the corruption of the young. It condemned his "violent hip-swinging" and "crude exhibitionism" and pronounced that "Tommy Steele became Britain's teenage idol without resorting to this form of indecent, short-sighted vulgarity."
It was, of course, never that potent. The young Cliff was a soft, unthreatening version of Elvis with none of his US counterpart's white trash undercurrent. Where Elvis was powerful, Cliff was sweet. Yet his early records, such as "Move It", were among the best of the early British rock 'n' roll.
How has he lasted so long. Others have survived but only by making more dramatic transitions: Tommy Steele become the latterday equivalent of a musical hall star; Adam Faith transmuted into a moderately successful character actor. Cliff, by contrast, kept up with the times, or rather just behind them.
In 1959 he and The Shadows found themselves in panto at the Globe Theatre, Stockton; in 1962 it was The Billy Cotton Band Show; in 1965 he compered Sunday Night at the London Palladium; in 1968 he made the first of two appearances singing for his country on the Eurovision Song Contest; in 1969 he starred on the Sooty Show; in the Seventies he became one of the first Western pop stars to appear behind the Iron Curtain; in 1980 he received the OBE, and in 1995 he took on the full status of Grade One listed pop star by singing with Vera Lynn at the VE Day anniversary celebrations.
How did he manage it? In the first (and best) of his movies, the satirical Expresso Bongo (The Young Ones, 1961, Summer Holiday, 1962 and Wonderful Life, 1964 all came later) he played an exploited young pop singer. Off screen, however, nothing could have been further from the truth - anyone less shrewd, determined or highly conscious of fashion could not have survived in the most competitive business in the world.
Nor is he a negligible musician. At a recent Greenbelt festival, the Christian equivalent of Glastonbury, he gave a solo performance of his big hits, accompanied only by his rather accomplished playing of his own guitar. Vocally he has a distinctively style of phrasing, so much so that even a professional cynic such as the pop writer Tony Parsons, listing Cliff's hits, concluded: "if you don't like at least some Cliff Richard, then you don't like pop music".
But in the end the key to his continued success is that slight frisson of safe sexuality. Cliff offers the expected quotient of on-stage pelvic thrusting but it is eerily sanitised and almost innocent. This ambiguity is at the heart of his persona. Cliff is the ever-available yet untouchable bachelor boy - never having suffered the setback of getting married he is at least still psychologically available to his fans. He is the fantasy lover who (apart from the wattling of his neck) did not grow old as those that they married grew old.
Most of his relationships have been platonic, according to his biographer, Steve Turner. Cliff has only had three serious romances: he lost his virginity at 18 to Carol Harris, the wife of his Shadows bass player, Jet; when 22, he had a passionate affair with Una Stubbs, then 24, on the set of the film Wonderful Life; and in the early Eighties he enjoyed a three- year friendship with former Wimbledon ace Sue Barker. "It was a doomed relationship. We attracted more attention than Charles and Di," Cliff complained when they split.
Repeatedly he has had to deny that he is homosexual - an allegation fed by the fact that he has lived for many years in Weybridge, Surrey, with his friend and manager Bill Latham, a former RE teacher. Bill, says Steve Turner, provides Cliff "with the emotional succour most men get from marriage". But then Bill's girlfriend, Jill, lives there too.
All of which, in this sex-mad age, is deemed to be rather strange and said to reveal that the man has something to hide. But not for Cliff the androgynous zone of ambiguous sexuality occupied by post-modernists such as Michael Jackson and Madonna whose sexuality self-consciously wavers across some genderless no-person's-land. Cliff is happy to prefer tennis to sex in real life and then offer ersatz sex on stage.
He sees no contradiction. For his Heathcliff the Brute he is not only putting on weight, he is perfecting moody and unshaven designer stubble. Not very Cliff, one interviewer suggested to him. "It's called Acting," the great man responded archly.
Some would say it's what he's been doing all his life. Others will just see Cliff in a Cloak and be happy to enjoy it.
Spot the difference: Cliff then (left) and now (right)