Ashwell House was quiet, the walls a startling salmon pink, and my first impression of Sam and Eileen was of a quieter version of television’s Trinny and Susannah, the hosts of What Not to Wear. Sam, dark-haired, in a high-collared red chiffon shirt; Eileen, blonde, made-up, in a V-neck lined with sequins. After a quick tour of the premises, ‘a good environment for clean-living students’, we settled in a common room for coffee and life stories. ‘I was an Anglican,’ said Sam, ‘at a little CofE school, but my experience of Anglicanism was of nice middle-class ladies in hats going to church on a Sunday, and I wasn’t satisfied. I thought, there must be an objective truth, and this isn’t doing credit to it. Then I met a Catholic girl who seemed to be cheerful and great fun, and started going to meetings with her. She was in Opus Dei, so I went to classes, then joined as a numerary.’ Simple! She smiles. How could you be sure this was your vocation? ‘I wasn’t!’ said Sam. ‘It’s more like being in love and deciding to get married — everything points in the same direction, it all makes sense and there’s a feeling of tremendous happiness.’

After admission to Opus Dei, there’s an order to a numerary’s day — morning prayers, daily Mass, daily rosary and an intense focus on hard work. They also hand money over to the movement. How much? I asked Eileen. ‘It’s up to you, up to your conscience,’ she said. ‘A peasant family, for instance, might give two carrots. You use the money that you think you need and give the rest. A good rule of thumb for what to spend on yourself is, “Would a mother of a large, poor family buy this for herself?”’

Eileen’s story is much the same as Sam’s. ‘Though I’d always wanted to get married and have children, I knew at 17 that my vocation was to be a numerary. I felt, unfortunately, this is what God wants me to do! It fitted like a key in a lock.’ But didn’t your parents mind? ‘Mine were very unhappy,’ said Sam. ‘They said, “You can’t leave home until you’re 21.”’ But did you? ‘Yes.’

‘Opus Dei doesn’t try to alienate you from your parents; we obey the fourth commandment,’ said Eileen. ‘But a lot of members join when they’re young and so the parents are overprotective.’ Both women seem baffled by this. ‘The only explanation I can give is that parents don’t trust the judgment of their children,’ said Eileen. ‘It’s odd, because it’s not as if the child has started taking drugs; they’ve just decided to give their life to God.’

‘Would you like to meet some of our young members?’ said Sam, and then followed an odd hour interviewing supernumeraries (the marrying kind). First up was Melissa, green cardigan and suede boots, with a breezy public-school manner.

‘I liked Opus because I met people who were young, vibrant, sporty,’ she said. ‘To be honest, all the Catholics I’d met before had been old and nice but, you know, not like me.’ She laughed. ‘Now if I feel like a big fat load of zeros, I remember I’ve got Christ in front of me and I can do anything!’