While adultery in the age of equality is committed by similar numbers of both sexes (most men are cheating with women, says Andrew, not men), the majority of the group is female. The hurt is red raw for some.

Lucy, 33 – elegant and witty – rages, “One day we’ll die, and we’ll have wasted all this time and energy into analysing this – when before we were just living!” Even as the group hoot with laughter, they nod in agreement, and their underlying sorrow is palpable.

When your spouse cheats, there’s a need to compare notes with fellow members of this unhappy club, because family take sides – unhelpful if you want to save your relationship – and friends either try to bustle you into divorce or stir up trouble.

Sarah, 55, is still reeling from the comment of a so-called well-wisher: “How the mighty are fallen.” Andrew says: “Friends often try to persuade you to do what they did, to reassure themselves that they made the right choice. Also, if you constantly canvass friends, how can you know your own mind? One person said, ‘Well, my hairdresser says…’ I wanted to weep!”

To avoid starting the session with an embarrassed silence, everyone pairs up to share their current concerns. This week, it’s how to deal with the rollercoaster emotions (his and hers), and to what extent to protect yourself, or acquiesce, if your errant spouse shows signs of wanting to rebuild intimacy. There’s talk of “the dance of pursuit and retreat”. Andrew jots the questions on a whiteboard, and gently facilitates the discussion. As he says, the kicker of infidelity is that “the person who considers themselves to be the most hurt has to do the most work”.

Mark, 47 – a high-earning Don Draper lookalike – confesses that, as a man, he had no idea how to cope with the maelstrom of emotion that followed his wife’s affair: “You hit us in the face, we can process a bloody nose,” he says, “that’s fine. But the emotional side of things, we don’t know how to process. If you turn to friends, you get the typical male response, ‘You’ll get over it.’ You feel helpless. You want to escape this loneliness, this ‘Woe is me.’”

But he just couldn’t. “I lost a couple of years of my life. I didn’t know what was real, what wasn’t. My wife was in denial, and defensive. She blamed me, and asked me to move out. For months, I slept on couches, and I couldn’t work. You fall to pieces. When your partner has an affair, you wonder, ‘What’s wrong with me?’ ” He adds: “Infidelity is rife but it is a taboo subject. This support group brings empathy. You’re not alone.”

Most members have had couples’ or individual counselling, but you sense that attending this group is salvation. They offer each other comfort and hope. Celia says that in the months following the affair, she and her husband would go to the beach. “We’d walk, talk and swim. We’d fly a kite. It was our link to our past – what we used to do – and our link to our future.”

She adds that negotiating her relationship now is “like flying a kite against the wind. When you’ve got the wind behind you, you fly with it, and when the wind drops, you try to get the air back in the sails.”

Meanwhile Andrew, who has specialised in infidelity for 30 years (I may have phrased that clumsily), has unparalleled insights into the psychology of betrayal; its genesis and consequence, and how best to unravel its tangled web. If a couple wants to stay together, they face the Herculean task of excavating their relationship, to understand why the cheating occurred and ensure it doesn’t happen again. As Andrew puts it: “They have to drain the swamp.”

Much as we love a goody and baddy, he prefers not to assign blame: “I’m more interested in understanding the equation ‘problem + poor communication + temptation = infidelity’. If you just say ‘it’s all your fault’, you get stuck, because you are the powerless victim, and he or she is the ‘bad person’. Often, there’s a problem – he was made redundant, and they didn’t discuss it. Is that his fault because he was brought up not to admit to his problems, or his wife’s fault, as she didn’t listen? Your marriage got to the point where it was vulnerable. Then along came temptation.”

Andrew isn’t prescriptive, but when Lucy asks, in despair, “Will this infidelity sit quietly on our relationship for ever?” he replies: “Really painful experiences never leave you, but you think about them less.” While no one wants the affair to be their sole topic of chat with their partner (Andrew advises limiting its discussion to certain times) some women are frustrated because they can barely prompt their husbands – reportedly both ashamed and depressed – to say a word on this sensitive subject.

Andrew suggests the magic phrase: “tell me more”. Encouraging, he says, as it shows you’re taking his ideas seriously. Other tips include gravely repeating the last thing your spouse says (e.g. “it’s tough”), or nodding and staying silent – although Lucy says she asked her husband a question, silently counted, “And I reached 540!”

The session – gruelling, upbeat, passionate, exhausting – ends on a positive note. Celia tells us that she’s had a gorgeous family photograph made into a 500-piece jigsaw. Each time she feels that she and her husband are a little closer in understanding, she adds a piece. She says, with a wry smile: “I’ve got this lovely picture of our family, and I’m slowly putting it back together, piece by piece.”

Join infidelity support networks at:

• andrewgmarshall.com/infidelity-support-group
• savannahellis.net
• move-beyond-the-affair.com

Read: How Can I Ever Trust You Again? Infidelity: From Discovery to Recovery in Seven Steps by Andrew G Marshall (Bloomsbury)