Sarah Cassidy is the sort of no-nonsense, capable woman you might expect to find as headmistress of a primary school. But Sarah doesn't do children, and she doesn't do husbands either.
No. Sarah is 43, single and celibate - and determined to remain so. Each night she fastens a wire chain, known as a cilice, around her upper thigh.
The device has sharp prongs that dig into the skin and flesh, though generally it does not draw blood. To most women, it sounds a peculiarly masochistic practice.
Yet Sarah says it serves a very different purpose: suppressing her desires and atoning for her sins.
Quite what those sins might be it is hard to imagine. For Sarah is not just good, but very, very good. She doesn't drink, abhors drugs and has never had sex.
More than that, she is a senior female figure in Opus Dei, one of the most controversial forces in the Roman Catholic church. Portrayed as shadowy and sinister in Dan Brown's international bestseller The Da Vinci Code, the group has been accused of obsessive secretiveness, elitism, misogyny and criticised for its methods of recruitment.
But it is the 'mortification of the flesh' - a ritualistic form of self-harming practised by many Opus Dei members - that has attracted most widespread condemnation.
Now, in a bid to correct false impressions, Sarah has agreed to meet me to discuss what it is that attracts women like her to what seems such an austere and, frankly, painful expression of faith. I meet her with fellow Opus Dei member Eileen Cole at the group's £7million London headquarters on Chelsea Embankment, where Sarah now lives.
First, though, some background. Opus Dei - Latin for 'Work of God' -was founded in Spain in 1928 by the Roman Catholic priest St Josemaria Escriva. Its doctrine focuses upon the lives of ordinary Catholics, who are neither priests, nuns nor monks yet who believe that everyone should aspire to be a saint.
Today, the organisation claims to have 87,000 members worldwide, about 60per cent of whom live in Europe - among them, former Labour education minister Ruth Kelly. Membership is divided into different categories.
About 70per cent are so-called 'supernumeraries' - married men and women with normal careers. They contribute financially to Opus Dei, and though they are not formally required to practise 'mortification', many choose to do so.
More committed, though, are 'numeraries' like Sarah and Eileen, who pledge to remain celibate, generally live in special Opus Dei houses scattered around the world, and often work directly for the organisation.
Mortification is part of their daily routine, including use of the cilice and periods of fasting.
So every evening, just before she does the washing up, Eileen, 51, straps her strand of barbed wire round her leg and leaves it there for two whole hours, scratching at her skin and digging into the flesh.
It sounds agony, but she insists it's 'less painful than a bikini wax'. And besides, pain is the whole point.
'It's an easy way of knowing you're doing penance,' says Eileen, who lives in an Opus Dei centre in Ealing, West London. 'I wear mine above my thigh. If you go swimming, you don't want to leave a mark from where it has been.
'To be honest, it's the fasting I find most difficult.'
Still, many of us would struggle to comprehend what on earth drives two intelligent, articulate women like Eileen and Sarah to cause themselves pain on a nightly basis.
Perhaps understandably, given some of its rituals and strictures, the movement is often condemned as a cult. Certainly, Eileen's parents thought so.
Her mother is now dead and her relationship with her father remains strained. They couldn't understand how their only daughter, who had never given them a moment's trouble in her life, left home at 17 to join Opus Dei.
'My parents hated me joining Opus Dei. I think they'd have been happier if I'd run away and joined the gypsies. They thought I was joining a cult. They were terrified. Absolutely terrified.'
So how did she become involved with the group?
'I never went to a Roman Catholic school and had boyfriends from the age of 12, because that's what I thought you had to do,' she says. 'You really weren't cutting the mustard if you didn't.
'But I changed to an all-girls school to do my A-levels, and suddenly there was complete freedom. You didn't have to have a boyfriend or flirt all the time.
'One of the girls in the sixth form belonged to Opus Dei. I told her I was a Catholic but didn't know anything about it, so as well as partying all over London, I'd spend Mondays learning the catechism.
'And I just started talking to people about my faith. That's what evangelism is - spreading the word.'
Still, the early boyfriends, those teenage parties ... hadn't she excluded herself from becoming a senior official of Opus Dei - a role which demands celibacy? Again, Eileen forces a smile.
'I wasn't promiscuous and I looked forward to a relationship within marriage,' she says. 'As soon as I knew what it meant to become a numerary, it was like a lump in my stomach.
'Of course, it's a huge sacrifice from day one to make the decision. But you're doing it for the Kingdom of Heaven, which promises to reward you a hundredfold.'
Within months of discovering the movement, Eileen was whisked away by her spiritual mentors to Spain, where she spent three weeks praying and considering her future.
This was part of her training before she could become a full member of Opus Dei.
'I didn't speak Spanish, so had a lot of time to think and pray at the sanctuary,' she says. 'I understood that this was my vessel to come back to God.' It was here, too, that she began wearing the cilice.
'The mortification helps you to keep your passions under control and channel your energy,' she explains. Despite Eileen's devotion to the organisation, others have criticised Opus Dei for its methods of recruitment, which include 'love bombing' potential members with affection and praise.
The requirement for recruits to hand over large proportions of their income has also raised concern. (Today, Opus Dei is a huge cash cow for the Roman Catholic church, with tens of millions of pounds-worth of buildings around the world, funded by donations from members.)
Other questions include why Opus Dei members do not normally divulge their involvement, leading to a concern that the organisation is seeking to establish itself as a form of Christian masonry. Membership is growing at the rate of several thousand a year, with women being particularly targeted.
Sarah, it turns out, has been directly tasked with drumming up membership in Britain among young professional and married women. There's even a glossy Opus Dei magazine 'for and by young people', with articles such as 'six tips for the perfect picnic' and 'the internet detox'.
But this is no ordinary from of 'sisterhood', and certainly not an easy-going one.
Unmarried male and female 'numeraries' are segregated in the Opus Dei houses where many of them live, with only limited contact between the sexes.
There's also a subgroup of female numeraries known as 'assistant members' who perform the cooking, sewing and cleaning and 'serve' the men. Men never serve the women.
I find one assistant member at the Chelsea HQ, shut away in a little room with her head bent over a pile of mending. It doesn't look much like fun to me.
I wonder how an educated woman such as Sarah, who studied physics at Manchester University, can condone such inequality. Again, though, she speaks of 'God's plan'. Sarah was 19 years old and part-way through her degree when she decided to give her life to the organisation. Intriguingly, she was also in a relationship with her first and only boyfriend.
She maintains, though, that a life devoted to faith was always on the cards. Her mother and her uncle were both Opus Dei members.
'I'm not going to say I was running after celibacy, but it wasn't something that was so foreign,' she says. 'My uncle was a priest, and many of my aunts were nuns.
'I'd seen people take the same path from the age of nine or ten.
'But sex wasn't something that repulsed or frightened me. 'If I'd got married, my ideal would have been sex with that guy.
'When I was little, I always imagined that I was going to get married and have children. 'But this is the vocation that God gave me. It's such a gift, and there's so much love in there.'
Needless to say, Sarah's smiling. In fact, she's an enviably serene woman. The oldest of five children (with four younger brothers), she was nine years old when her mother joined Opus Dei.
'I was a rather intense, reflective child and not very attractive,' Sarah says. 'My parents [her father was a newspaper photographer] had a very deep relationship with God and passed that onto us. We used to pray every day as a family - say grace before meals and prayers before bed. We'd pray when my dad lost his job, or I didn't get the exam results I wanted, or my mother lost a baby.
'She lost quite a few babies. I just remember she wasn't well, she got better and the baby had gone. My mother always had this sense that God had a plan.
'Having a boyfriend wasn't as important to me as it was to everyone else. It just wasn't a ruling passion in my life. I wanted to be a pilot.
'I had my first boyfriend at 19, and it was when I was starting to go out with him than I began to realise I wanted something else.' The boyfriend, it turns out, was a fun, pleasant chap but not a Catholic.
'Yes I kissed him. There was nothing wrong with the relationship, but I was at that stage in my life when I was going over what God wanted me to do.
'He was very understanding when I reached my decision that it was the end of the relationship. And it was always very clear I wasn't interested in that experience.'
By 'that', she means sex.
'I didn't like the promiscuity I saw at university,' she says. 'I didn't want that for me. I didn't want that life. I rejected it as part of my lifestyle. I didn't want to be part of it.'
I cannot get over the small matter of the cilice - surely it's a seismic leap from eschewing promiscuity to self-harming in this way?
Sarah was 20 when she started wearing it. 'The first time you do anything that's not particularly pleasant, you don't like it. But over time it's just something that's there. The result of doing it is that you should be a much nicer person afterwards.'
Eileen adds: 'We live in such a materialistic, hedonistic society that people can't understand you'd actually make yourself a little uncomfortable to help you be more mindful of God.
'They'll understand if you go jogging and pounding the streets - which I think is disgusting - just because you want to be thinner, but they won't understand this.'
It's a fair point. After all, which is more peculiar or 'unnatural': women who endure the agony of, say, Botox injections or leg-waxing, in order to be beautiful, and those Opus Dei devotees who strap on a cilice as a sign of spiritual devotion?
Still, I can't help feeling that most women would consider it a strange God who requires them to do the washing up wearing a chain of barbed wire.
