I can't remember exactly when or why I started going but Tranquillity has become a regular part of my Monday morning for the past couple of years. Okay, so I've missed a few because of having to work, but if ever I am free and in Stratford between 10 and 11am this is most likely where you'll find me. If you're ever around at that time you should come too.

Tranquillity used to be called Stretch and Relax and when I'm describing it to my friends I call it Yoga for Plebs. It's a class held at Atherton Leisure Centre, just up the road from me, in which we do a lot of yoga breathing, a load of poses, and then a bit of relaxation.

Tranquillity couldn't be further away from Madonna-style yoga, or the yoga practised by supple young things in beautifully minimalist studios in the more upmarket neighbourhoods of our city. For a start it is cheap. I pay the full price of £2.90 for a one-hour session and it's the best money I spend all week. Secondly, the punters are decidedly unglamorous or hip.

Let me say a bit more about my classmates. The same faces turn up every week, plus a few new people. Of about thirty participants half are wiry pensioners, there are a handful of gym bunnies, plus miscellaneous oddballs, students and misfits. Most are black and Asian and all are women, give or take the rare man who ventures in to the class once every few months or so. We are "the ladies" and "the girls" as in "Alright Ladies, time to get on your feet," or "Come on girls and B R E A T H E!" or "Now ladies, I want you to clench and relax your toilet muscles."

Denny is the star of the show, she's our instructor. Now she is in the Madonna league. I can't believe her body, it's muscle and sinew and bone. She is super-strong and can do unbelievable things. It makes me feel humbled that a goddess like her should want to spend quality time with a bunch of creaky old farts like us.

It's hard for me to feel kindly about skinny athletic women, since their bodies are the standard by which fat folk like me are measured in the cultural eye. Women like Denny are the ideal, and I am at the other end of the spectrum, the very absolutely definitely not ideal. But you can't deny that she is marvellous, her suppleness is incredible. I am glad I can see the goodness in her body, it makes me feel one step closer to my personal utopia where all body difference is respected.

Anyway, we are all in love with Denny. She sees us at our most enfeebled and still makes us feel as though we can really do this thing and that, yes, we are actually getting better at it. We thrive on her exuberance and enthusiasm. She makes us feel blessed and special just for having a go at downward-facing dog, or warrior three, or lion, or frog, or pigeon pose. She is totally dedicated to us, a real professional, she pushes us and she's gentle. We are so lucky to have her and we know it.

We suck up thrilling bits of personal information about her: her chronic lateness; the fact that she didn't drink enough water when she was younger; the 17 years she spent as a secretary in the city; the weekend she spent with ultra-tough fitness enthusiasts (me: "freaks") whose wrists, she said, are probably weaker than ours because they couldn't do the dog. Best of all was when we found out that it was her 40th birthday, no one could believe that she wasn't in her twenties. We all chipped in and got her a present and a card.

I'm as intrigued as anybody and I wonder what Denny's like in real life. I imagine her stuck in a traffic jam, trying not to succumb to negative energy. I think of her practising her yoga moves in her front room at home, or buying more mail-order new age yoga muzak tapes, or shopping for exercise outfits, or hanging out with her friends (which is where my imagination stops - I can't envision who her pals might be).

Denny's a bit of a yoga hippy. She tells us to focus our third eye when we are trying to balance on one leg. She talks about auras and energy. Sometimes we do a move that requires us to "thank the earth" and "salute your god, whoever that may be." I salute rock and roll. This hippy talk would severely get on my tits out in the real world but at Tranquillity it makes me smile, it makes me happy.

For a while we were a class in search of a venue. As the number of Tranquillity fans grew we were moved around the leisure centre from studios that were so tiny we had to sit in a certain formation to avoid knocking over our neighbour, to the indoor bowls hall, where there was open hostility between us and the noisy bowlers, who had "negative vibes," according to our leader.

As hippies do, she went travelling last year for a few months. We had a replacement teacher who was so tough and relentless that it decimated the class. I think there were cheers when Denny returned.

I love Tranquillity. I'm always scared before I go that I won't be able to do it, that it'll be too hard for me this week. I don't say much in the class. I try and do my best. I always have a go and I'm always loose and warm and relaxed and filled with happiness afterwards. I love being with a group of friendly women who I might not otherwise get the opportunity to meet. I'm the quiet one, it's like I'm taking a holiday from being me. But the best reason to go is that there's always a chance something truly extraordinary will happen whilst I'm there, something so lovely that it'll carry me through a week of drudge, or depression, or grief or sadness.

I remember one time Denny got carried away with affirmations and managed to get every single person to say alone to the class: "I am loved." It was a beautiful and moving experience. On another occasion we worked in pairs, helping each other to balance whilst we attempted princess pose. I was with a woman who must have been in her sixties, she was small and a bit podgy, all snowy hair and bony feet. I held her hands as she stood on one leg, bent forward, and unfolded her body in a breathtakingly elegant arc, I would never have guessed that this woman had such grace within her.

But the most remarkable Tranquillity moment came last winter. It was a bitter day, a week into the American invasion of Iraq, and everybody felt numbed and distressed by the war. We sat in a circle in the Atherton bowling hall and went through our moves. When it came to the end, to the relaxation part of the session, we simply chanted. We chanted for the people of Iraq, we hummed and breathed and sent all our love and vibrations to the people who were getting battered. It was corny, that's true, and it would never have changed the course of events, but it was done with love by a group of humble, ordinary people who don't feature on our government's radar; we sent out warmth to our sisters and brothers when every other form of protest and show of war-disgust had failed. It was the only thing left for us to do.