If I suggested getting rid of some slightly troublesome plant, she would say quickly: but I love it so!

Penelope Mortimer has a problem. Anna Pavord tries to solve it

"When I moved from Gloucestershire in 1991, I blithely set out to make a country garden in Willesden, north London. It's worked, I think, but in the meanwhile I got rid of a chunk of lung and suddenly a 'country garden', even though relatively small, is impossible to deal with. London gardeners, at a minimum of pounds 10 an hour, are out of the question (anyway, most of those I have tried are cowboys, hardly capable of mowing the grass). I know the lawn and the borders have to go, if I'm not to live in a state of chronic frustration, but what to put in their place?

"The garden is south-facing, lots of trees, about 100ft squareish. I can't bear the idea of it all being paved, with those dreary raised beds for the aged. How do I cope?"

The letter came from Penelope Mortimer, author of The Pumpkin Eater and My Friend Says It's Bullet Proof. She discovered gardening when she moved to Gloucestershire about 20 years ago and has remained a gardener of the most passionate kind ever since. The letter seemed to be saying that she was looking for ways to simplify the garden she had made in London over the past four years - she will be 77 this year - but when we met I did not get the impression of a woman who was ready to give up anything at all.

If I tentatively suggested getting rid of a slightly troublesome plant, she would reply quickly, "But I love it so." A proposition for simplifying a border would be countered with an immediate, "But I love doing it. That's the whole point." And her chin would tilt a little further upwards as she reached for another cigarette.

Since I agreed with her wholeheartedly, and respect the indomitable spirit that her replies reflected, we talked more about complicating the planting schemes even further than we did about reducing them: what shrub to use in a space at the back of the left-hand border, whether a monkshood would be the right plant to take over from the campanulas. We decided that it probably was and then had an equally long debate about whether 'Newry Blue', an almost navy-blue aconite, or the wider branching 'Spark's Variety' would be the best bet. Her reference books all had the smudged, sun-bleached look that spoke of hours spent outside rather than in.

The garden, almost as wide as it is long, is an unusual shape for London. A conservatory at the back gives on to a terrace, then lawn with a big, shimmering aspen, Populus tremula, on the right-hand side. Ash, elder and the like screen the back boundary and the two fences on either side are thick with ivy and climbers. From the terrace you see nothing but green.

The most effective feature is the central arrangement of two curving, C-shaped beds each about five feet wide, which almost enclose a central roundel of lawn. This is about 12 feet across and is decorated with a stone statue. A grass path leads down the middle, separating the two curving beds and joining the lawn on the far side. Stepping stones take you towards the back of the garden where there are fruit trees, and a big centrally placed iron arch smothered in the almost evergreen climbing rose 'Adelaide d'Orleans'. This is a friendly rose, bred in France in 1826, with few thorns and clusters of small pale-pink flowers. Twined through it is an equally vigorous clematis, a Viticella with double purple flowers.

In the back right-hand corner of the garden, the ground rises in a way that suggests it was probably once a rubbish dump. Crowning the spoil heap is a vigorous young loquat. At least, that is what I would call it. Ms Mortimer grew it from a cutting and said it had been given to her as nespole, a name she had been unable to track down in any of her books.

In the other corner was a young golden robinia, shooting for the sky in a most alarming manner. It looked as if it was trying to outpace a young ash that was leaning into its airspace. The trunk of the ash had forked quite high up and I suggested to Ms Mortimer that she would be helping the ash and the robinia if she took out the left-hand fork entirely. The ash would then grow on with a single leader and the robinia might be in less of a hurry to find living room in the clouds.

The ground under the robinia was carpeted with a mat of brightly variegated ginger mint, which reflected the colour of the robinia's leaves, with golden creeping jenny and big hairy clumps of pulmonaria. A pretty parsley- leaved blackberry was propped against the robinia's trunk. It would probably be safer trained against the ivy on the fence.

Running along the left-hand boundary of the garden was the biggest border of all, meeting the lawn in a gently curving line and coming out almost as wide as the long low extension to the house on that side. Crambe, grey- leaved buddleia, a superb melianthus with steely blue-green leaves and a waxy texture and the grey-leaved rose R. glauca fill the back of the border. In the foreground are intricate plantings of snapdragons, Geranium renardii with leaves as rough as sage and ballooning seedheads of love- in-a-mist.

"I never give it a chance," said Ms Mortimer, looking critically at the border. "I'm always moving the plants about. They never have time to get to any size." Seizing the moment, I suggested that perhaps this would be one fairly painless way of cutting down on the work. Leave the border be. There were some good plants in it. Given the chance to settle, they would spread their wings gracefully and obviate the need for interplanting extras.

The most obvious candidate for change might seem to be the central curved beds. So simple, you would think, to grass them over. But if this were my garden, those beds would be the last things to go. The strength of the centre holds the rest of this garden together. Too few gardens are planned from the centre out, as this one is. Most people garden round the edges of their plots so that the middle becomes not an integral part of the design, as it is in Ms Mortimer's garden, but just the space that is left over when everything else has been sorted out. When this happens, the edges of the garden fail to connect with each other. The garden becomes like a vortex with a consequent feeling of uneasiness on the part of the person who is looking at it.

But the big border could easily be simplified with little loss. It might even enhance the garden, as bulkiness in this area would give a less distracting background to the intricacy of the central borders. Ms Mortimer could also get rid of some of the many small pots crammed on the terrace. She could eliminate the narrow border on the right-hand side of the plot and grow only wall shrubs there. She could garden less intensively at the back of the plot, where she has only just finished making a new border of bamboo and geraniums.

But I do not think she will. She knows she ought to be sensible, but the most brilliant thing about her is that she is not. She is unreasonable, impassioned and she needs to garden.