The garden in which we're sitting this lunchtime isn't any old garden, it's London's oldest botanic garden, conceived in 1673 as a place where aspiring young apothecaries could grow and study medicinal plants, before converting them into pill and liquid form and inserting them into their patients.

Around us, just outside the 17th-century house, the dozen or so round wooden tables seem to be occupied entirely by the audience of Gardeners' Question Time. The ladies wear crisp cotton blouses and cashmere cardigans and carry their handbags like trugs; their elderly husbands are raffishly arrayed in yellow sweaters and cerise trousers, as if preparing for a hard afternoon's avant-garde plantsmanship.

Behind us is the garden's handsome tropical-plant greenhouse, full of alarmingly phallic cacti. Before us is a long, trimmed lawn, across which babies and toddlers are rolling and tumbling in the tranquil afternoon breeze. In the distance the Thames sulkily twinkles.

This is a lovely, peaceful setting for lunch, disturbed only by the overhead drone of post-volcano aeroplanes. Why are the best gardens in London – Kew, Petersham, Chelsea, Richmond – all sited in the western metropolis, on the Heathrow flightpath? Couldn't their founders have seen runways coming?

The Tangerine Dream is a café/ restaurant with a small kitchen, a staff of five, modest ambitions and a marvellously sure way with food. The menu is small, offers no starters and far too many cakes for pudding but has a nice, relaxed feel about it. We ordered a mixed salad to play with, and were rewarded with a cornucopia of tastes: endive with blue cheese, edamame beans with prosciutto, Beluga lentils, golden and red beetroot, feta cheese, new-potato salad with wilted garlic leaves and salsify. Even a salad-dodger like me could tell this was a bit special. The combination of pulses, greenery and pungent cheese was fabulous. (I may have to look into this salad thingy. I'll soon be bringing you news about a sensational radicchio I've discovered in Kent.)

The five main courses were slightly predictable but welcome none the less: confit of duck with celeriac mash; salmon en croute with stem ginger; seabass; steak; and goat's cheese tart. My steak was served in the Tuscan tagliata fashion, lightly charred and sliced sideways, with a Portobello mushroom and a touch of oil. It was delicious. Even its tepidness seemed right for an al fresco lunch. The horseradish was cut too liberally with cream, in my view – I like having my throat scorched – but the tiny new potatoes were wondrous.

Madeleine's main course was as perfect a duck confit as I've ever come across – the skin super-crispy, the leg meat rich and fibrous, the mash faintly redolent of truffles. "It's a winter dish, not a spring one," she said, "but I really don't mind at all." The sea bass, though fresh and summery, was a little under-flavoured. "I bet it's not wild sea-bass," commented my pal Laurence – and we sat silently wondering what a truly feral, snarling, livid sea-bass might taste like...

There's something about lunch outside that makes a glass or two of wine seem mandatory. Something light and scarcely alcoholic, like a Pinot Grigio. Amazing how, when you're in the open air, it slips down without touching the sides. Also amazing is the decision by the Tangerine Dream people to serve only cakes for pudding. It means that, rather than tackle your usual crème brûlée or rhubarb crumble, you're forced to try the coffee and walnut cake, the orange polenta cake, the Sacher torte and the Amalfi lemon tart. They were very fine, but the tart had a sweet, velvety curdiness that pitched me back to childhood, when lemon curd came in jars, homemade by your lesbian auntie in Maidstone.

With main courses costing an average of £10, the puddings £12 for a selection of cakes and the wine £15, lunch for three came to an astonishingly cheap £70. Tom Aslanian, our waiter, turned out to be the ex-general manager of Bibendum, and was charm on legs. David Hughes, the chef, dropped by to explain how he wished he could offer more dishes, but his kitchen is the size of a matchbox. We ended lunch sighing with pleasure that such a lovely garden restaurant existed in the heart of London. "And we haven't had to trek all the way to Sissinghurst to get this kind of atmosphere," said Laurence. I agreed. The garden's 5,000 plants waved cheerily in the breeze. The huge Victorian ferns moved as if operated by a punkah wallah. More babies tumbled blithely on the immemorial greensward. It was all lovely.

"The only thing that's spoiling this moment," I said, "is the guy over there with the strimmer." The others looked at me. "You can't complain about there being gardeners here," said Laurence. "They're the reason this whole place got started..."

Tangerine Dream Café, Chelsea Physic Garden, 66 Royal Hospital Road, London SW3 (020-7352 5646)

Food 3 stars
Ambience 5 stars
Service 5 stars

About £50 for two, with wine. Plus £8 entrance fee to the garden

Tipping policy: "No service charge. All tips go to the staff"

Side Orders: Outside inns

Ferryboat Inn

Helford Passage, nr Falmouth, Cornwall (01326 250625)

Make sure you book yourself a river terrace table at this atmospheric pub; the seafood here is ultra-fresh.

The Samling

Ambleside Road, Windermere, Cumbria (01539 431 922)

This hotel beside Lake Windermere has a lovely outside terrace, and the restaurant here recently won a Michelin star.

Auberge Du Lac

Brocket Hall, Hertfordshire (01707 368 888)

This gorgeous lakeside restaurant specialises in dishes such as seared scallop with pork belly and black pudding.

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