There’s a cackling coming from under the Parliament Hill bandstand, but there are no jokes being told.

The source is a class of laughter yoga, a “wellbeing workout,” combining exercises and childlike playfulness with yogic breathing.

Deceived by the sunshine, I arrive ill prepared for the harsh winter wind. Our instructor, Mary Adshead, insists we keep warm. She has a bag full of sweaters to help. A fetching purple and pink cropped knit is fished out for me.

Mary has been teaching laughter yoga for six years. “We choose to laugh in laughter yoga,” she tells me. “It’s not so much entertainment as something you do for health. And the more you do it, the easier it gets to laugh and to see the funny side of things.”

Laughter yoga is the brainchild of Dr Madan Kataria, an Indian physician known as “The Guru of Giggling,” who developed the technique while writing an article titled Laughter – The Best Medicine. What began in 1995 with a class of five people in a Mumbai Park has now spread to more than 16,000 laughter clubs in 72 countries.

We introduce ourselves with a “full-on laughter handshake”. There’s Renee from France, hunched over a walking stick but full of joie de vivre. Lisa, a Buddhist who says laughter yoga helped her through some dark times has been absent recently as she normally chants for world peace on Sunday mornings. She’s brought along another newbie, her friend Adam. “It sounded like a laugh,” he says. We warm up and stretch out our “laughter muscles” with a few guffaws. Clapping in rhythm, we chant in unison: “Jolly good, jolly good, yaaay!” It’s a refrain we return to throughout the hour.

Forming a circle we take turns to run into the middle and introduce ourselves. Mary says the exercise is inspired by the story of Spartacus, whose fellow slaves all stood forward to shout “I am Spartacus” when a Roman general demanded the gladiator was handed over to be executed. “I am Sue hippopotamus!” declaims one. Sue is a big fan of screaming therapy, and Mary welcomes her invitation to emit a mutual roar. Last week, Sue convinced a Waitrose cashier to let her lead the checkout queue in a scream. “The cashier said after, ‘could you come in once week?’”

“Every little thing will be alright,” we sing, before revving up invisible motorbikes with laughter, and gleefully riding them around the bandstand. We mime our preparations for a date, brushing our hair while giggling at the reflections in our hands.

Shy sniggers are replaced by raucous belly laughs as we discard our self-consciousness. Our egos are boosted by shared compliments: “Hello you glorious god!” I’m greeted. “Hello you gorgeous goddess!” I reply.

The say laughter is the best medicine, and science backs them up. A University of Maryland study found that humour can protect against heart disease. Laughter yoga is said to reduce stress hormones and stimulate endorphins and the immune system.

We cover the whole spectrum of laughter, from “ho ho hos” to wicked witch cackles, infantile giggles, and silent chuckling.

“I don’t get it,” says Adam. “Whether you get it or not” Mary responds, “You can still laugh!”

The technique is known as “voluntary laughter”. But soon I can’t differentiate between the fake and the authentic laughs.

“We pretend with enthusiasm but it often becomes real,” explains Mary. “But it doesn’t matter if it doesn’t because the body can’t tell the difference.”

We end with a tittering conga. My natural cynicism had made me an unlikely convert, but I leave with a smile on my face and energy in my body. Those cathartic chortles provide a welcome escape.

Laughter Yoga takes place on the first Sunday of the month at 11am on Hampstead Heath. Donations to charity. Telephone: 020 7609 6145.