As tales from the royal nursery show, when a couple is at war, nanny is in the front line. To survive, she must learn the art of diplomacy. Rule one: don't take sides
0nce an estranged husband has his hooks into the kiddies, even on a part-time basis, will their relationship with their grieving mama ever be the same again? Probably not. No wonder mothers are prone to conjure up the spectre of a wicked snake-in-the-grass nanny, employed to insinuate her way into the childrens' innocent infant hearts and wheedle their affections away from where they rightfully belong.

The phenomenon has had a very public airing in the current press and dinner-party speculation about the spat between the Princess of Wales and Alexandra "Tiggy" Legge-Bourke, whose job as assistant to Prince Charles includes helping to look after the two royal sons.

At first the aggro was confined to nursery matters. Snarling "I am the boys' mother, thank you very much!" Di is reported to have set up a hedge of elaborate rules to segregate Tiggy from the princes, including forbidding her to supervise bathtime or read bed-time stories; she also threw Tiggy out of the royal car and made her travel with the other "servants".

Tiggy, who has been known to refer yuckily to Harry and Wills as "my babies", was soon portrayed retaliating with a jibe at Diana's maternal style. "I give them what they need at this stage - fresh air, a gun and a horse," she sniffed. "She gives them a tennis racket and a bucket of popcorn at the movies."

All this escalated rapidly; newspapers got involved in the feud; the mother apparently then insulted the nanny at a party and the whole sorry business ended in a welter of lawyers, injunctions and atrociously embarrassing publicity.

But it seems the two-faced, conniving-nanny figure is likely to be a figment of an over-anxious mother's imagination. In fact, many nannies seem very far from enthusiastic about the idea of trying to establish themselves by worming their way into the affections of their charges. A post with a family headed by a lone father is seen as a difficult proposition.

"I had a job with a father who had custody of his two children, and it was a nightmare," says Carol, 31. "I only lasted four months. The children were traumatised by the divorce and monstrously badly behaved, and the father was quite traumatised too and would hit the roof at the slightest thing. He would get virtually apoplectic because one of the kids complained about not getting fish fingers for tea." So, you didn't feel like becoming their lovely new mummy? "No! I was out of there as soon as I found another job."

In some cases, the nanny-father relationship can work out very well. "Fathers are more easy-going," says Clare, 26. "In my last job, the father worked from home. I would often have lunch with him - it was a really nice atmosphere. We would talk about everything, and he knew all my nanny friends. If I had a problem I went to him, not to her. But at the end of the day he was my employer. We only had one row in the two-and-a-half years I was there, but it was far more upsetting than it would otherwise have been because I had started to think of him as a friend."

Though, of course, some nannies may become queen of dad's heart and live happily ever after, salacious stories are few and far between. Clare couldn't think of any examples among her friends of nannies who had started an affair at work. "But I do remember one family where the mother started doing her hair like the nanny, and buying the same clothes - spooky. I'd have been very worried if it'd been me. Though this nanny was very beautiful - lots of mothers don't want beautiful nannies. It's an advantage if you're pig-ugly, actually."

Other fathers are just plain weird. "I recently worked in a family where the mother was a real career woman and the father was supposed to be the main parent - he was there more often," says Victoria, 23. "He didn't even say hello in the mornings - it was as though I was a servant. He took no interest in the children, who were only small. He'd rather stick them in front of the video and sneak off for a fag than spend any time with them. They got all their love from me - I wanted them to feel loved and wanted. If they needed anything they would come to me, poor little boys. The parents were a couple that should never have had children." So did she slither in and take over the children's affections? "You get attached to the children, of course, but it's a business relationship. I still see the boys, but there is always that bridge or distance between the family and you."

A father coping alone with children will require high-calibre help. Vicky Barker, manager of Nannies Plus and Canonbury Nannies in Finchley, estimates that around one in 10 of her bookings is for a lone father. "There have been far more over the last year or two, maybe from divorce, or because the mother has passed away, or sometimes the father has won custody of the children. I would send a nanny who would fit in with a single father. She would be someone mature in outlook, level-headed and experienced, not one whose eyes light up and who says 'I hope he's loaded!' - that has happened!

"You'd think it would be prime time for the nanny to step in and grab the father, but very often they have girlfriends lurking. It's rare for a nanny to have an affair with the father, and it actually happens more often with a couple - a single father has far more pressures and probably hasn't even got the time to think about it."

"Lone fathers are often in a difficult enough situation without adding further complications," adds Hilaire Gomer, author of The Good Nanny Guide (Vermilion, pounds 11.99). "In that situation, many nannies love the responsibility, and one with gumption and go will get on with it. Sometimes the nanny is thrown in the deep end - many of them are very young, and some need a good deal of hand-holding and attention. But in nine of 10 situations it all works out very well."

But if it doesn't, the poor nanny may find herself in a no-win situation. The harder she works and the better she does her job, the less likely the lurking mother is to appreciate her efforts. As Hilaire Gomer points out: "Princess Di won't be terribly enthusiastic about giving Tiggy a reference."