What could be more British than nosying around someone else’s private property while munching on a slice of cake? The National Garden Scheme allows you to do both, opening up people’s back gardens to the public and offering them a lovely homemade afternoon tea while they’re at it. I grew up poring over the pages of its famous Yellow Book of open gardens, envying the fat borders of geraniums and delphiniums in the rural area where we lived. But the NGS doesn’t just do big walled gardens and sweeping lawns; it has a London Yellow Book, too, and while the gardens are far smaller, the plants, the cakes and even the wine that some homeowners offer are no less lovely.

‘Lovely’ really is the best word to sum up the whole scheme. It is lovely to wander through the different rooms that most of these tiny gardens are split up into, finding new planting combinations and cultivars you’d never heard of. It is lovely then buying some of those plants, raised from cuttings by the garden’s owner. It is lovely meeting that owner, who is always glowing, not so much from the heat of the day but from the pride of being told that the garden they’ve bustled around all year in preparation for this day is just, well, lovely. Everyone attending is in a good mood, or even a little excited as they’ve just discovered Campanula ‘Pink Octopus’ (a hideous name for a curious little plant with thin, fleshy spotted petals).

The NGS is famously stringent when it comes to admitting gardens to its Yellow Book. The London gardens have to meet all those standards but with far less space than their country cousins. They are often no bigger than the show gardens at RHS Chelsea, but NGS plots require so much more thought and skill than those professionally designed gardens. A real garden is a living tapestry woven by real people as they go about their normal lives. It can’t just be filled with the same combinations of alliums, foxgloves and young bronze fennel that Chelsea commentators annually declare to be ‘very now’, ignoring the fact that those three plants are always in flower when the show opens, and are therefore an easy addition to the design. It can’t, in fact, just rely on combinations of flowers that work for one week of the year before collapsing into a mess of dying foliage. At 57 St Quintin Avenue, for instance, Harvey Groffman has put as much effort into the combinations of foliage as he has with his very bright hanging baskets. He has made one spot very colourful indeed without any flowers at all: a lime green Choisya ‘Sundance’ next to a dark, almost black-leaved Pittosporum tenuifolium ‘Tom Thumb’, and underplanted with flame-yellow-and-green-leaved Coprosma, ochre Heuchera and a silver-leaved Lamium. Even better: the first three plants in that list are evergreen, which means this garden doesn’t turn into bleak twigs when it isn’t NGS season.

Some gardens have a theme. ‘Let’s sit awhile,’ says Hazelanne Lewis, leading me to a bench that is almost drenched in the essential oils floating out of the lavender, sage and oregano planted around it. Her garden on Christchurch Hill, Hampstead, is devoted to plants used by Tudor herbalists. She still uses some of the plants as remedies: ribwort plantain, for instance, apparently helps draw infections from wounds. Eryngium is a beautiful plant that’s very useful for adding a sharp structure to a border, as it has bright blue-purple young foliage and purple flower heads surrounded by spines. It turns out, though, that this little blue plant was also once regarded as a little blue pill and used to treat erectile dysfunction.

Of course, because these are real gardens tended by amateurs, not everything works. Groffman’s patch of poppies and cornflowers that he sowed as a memorial for the first world war is looking dry and scrappy after weeks of heatwave. But the stunning thing really is how rare it is to see a failure in any NGS garden. At 81 Camberwell Grove, I caught the owner picking up a single leaf that had fallen on one of the patio areas. The garden wasn’t at all regimented but everything had a place. The different sections on this plot spilled into one another, going from a sunken area at the back of the house where creeping rosemary (Rosmarinus officinale ‘Prostratus’) forms a green waterfall down the walls, to a paved area crammed with pots of cute annuals and self-sown Erigeron and Alchemilla popping up between the slabs, to another planted border opposite a fish pond (admittedly difficult to see because there were always excited children peering into it), through to another section before the final patio area and a beautiful garden room with terracotta-painted walls and Pelargoniums sunning themselves. Sculptures pop their heads up between the plants, but these weren’t the big dominating sculptures of a flower show: they were the sort of thing you’d find in a real garden that has grown slowly over the best part of a decade.

Eighty-one was a colourful garden, with deep purple Salvias, carmine-flowered daylilies and orange Echinacea. Next door, the owners of 83 Camberwell Grove had opted for a gentler palette, although they, too, were keen to stop their immaculate garden from appearing to take itself too seriously. Next to two towering yew obelisks that open up a window into the largest border is a stone trough of flowers usually considered far too tacky to live in any self-respecting garden: Busy Lizzies.

The largest border, which is only initially visible through those obelisks, starts with the fattest-flowered hydrangea imaginable — a white cultivar called ‘Annabelle’ — and sticks largely to a theme of whites and purples. There are little aberrations to this, such as a variegated figwort, which has tiny red flowers, but they add a little more depth to the border. Again, foliage is important: the pond opposite this border provides the bright colours with orange, yellow and shubunkin goldfish, while above the palette stays soothing and cool, with hostas, a spiky equisetum and aruncus.

Eighty-one was one of the best gardens I’ve ever visited. It had a range of heights, scents and textures. You could also tell that it looks splendid in spring, as there’s a magnolia in one part, and two acers suggest that autumn is pretty fabulous too. But then again, it might have been the fact that visitors were also offered sparkling wine as they walked around. Either way, the whole scheme is excellent, inspiring and, of course, really rather lovely.