'Mummy, you used the F-word,' Jochen said, his voice softened with stern reproach.

'I'm sorry, but that man really annoyed me.'

'He was only trying to help.'

'No, he wasn't. He was trying to patronise me.'

Jochen sat and considered this new word for a while, then gave up.

'Here we are at last,' he said.

My mother's cottage sat amidst dense, thronging vegetation surrounded by an unclipped, undulating box hedge that was thick with rambling roses and clematis. Its tufty hand-shorn lawn was an indecent moist green, an affront to the implacable sun. From the air, I thought, the cottage and its garden must look like a verdant oasis, its shaggy profusion in this hot summer almost challenging the authorities to impose an immediate hosepipe ban. My mother was an enthusiastic and idiosyncratic gardener: she planted close and pruned hard. If a plant or bush flourished she let it go, not worrying if it stifled others or cast inappropriate shade. Her garden, she claimed, was designed to be a controlled wilderness – she did not own a mower; she cut her lawn with shears – and she knew it annoyed others in the village where neatness and order were the pointed and visible virtues. But none could argue or complain that her garden was abandoned or unkempt: no one in the village spent more time in her garden than Mrs Sally Gilmartin and the fact that her industry was designed to create lushness and wildness was something that could be criticised, perhaps, but not condemned.

We called it a cottage but in fact it was a small two-storey ashlar house in sandy Cotswold stone with a flint tiled roof, rebuilt in the eighteenth century. The upper floor had kept its older mullioned windows, the bedrooms were dark and low, whereas the ground floor had sash windows and a handsome carved doorway with fluted half-columns and a scrolled pediment. She had somehow managed to buy it from Huw Parry – Jones, the dipsomaniac owner of Ashton House, when he was more than particularly hard up, and its rear backed on to the modest remnants of Ashton House park – now an uncut and uncropped meadow – all that was left of the thousands of rolling acres that the Parry family had originally owned in this part of Oxfordshire. To one side was a wooden shed-cum-garage almost completely overwhelmed by ivy and Virginia creeper. I saw her car was parked there – a white Austin Allegro – so I knew she was at home.

Jochen and I opened the gate and looked around for her, Jochen calling, 'Granny, we're here,' and being answered by a loud 'Hip-hip hooray!' coming from the rear of the house. And then she appeared, wheeling herself along the brick path in a wheelchair. She stopped and held out her arms as if to scoop us into her embrace, but we both stood there, immobile, astonished.

'Why on earth are you in a wheelchair?' I said. 'What's happened?'

'Push me inside, dear,' she said. 'All shall be revealed.'

As Jochen and I wheeled her inside, I noticed there was a little wooden ramp up to the front step.

'How long have you been like this, Sal?' I asked. 'You should have called me.'

'Oh, two, three days,' she said, 'nothing to worry about.'

I wasn't feeling the concern that perhaps I should have experienced because my mother looked so patently well: her face lightly tanned, her thick grey-blond hair lustrous and recently cut. And, as if to confirm this impromptu diagnosis, once we had bumped her inside she stepped out of her wheelchair and stooped easily to give Jochen a kiss.

'I fell,' she said, gesturing at the staircase. 'The last two or three steps – tripped, fell to the ground and hurt my back. Doctor Thorne suggested I got a wheelchair to cut down on my walking. Walking makes it worse, you see.'

'Who's Doctor Thorne? What happened to Doctor Brotherton?'

'On holiday. Thorne's the locum. Was the locum.' She paused. 'Nice young man. He's gone now.'

She led us through to the kitchen. I looked for evidence of a bad back in her gait and posture but could see nothing.

'It does help, really,' she said, as if she could sense my growing bafflement, my scepticism. 'The wheelchair, you know, for pottering about. It's amazing how much time one spends on one's feet in a day.'

Jochen opened the fridge. 'What's for lunch, Granny?' he asked.

'Salad,' she said. 'Too hot to cook. Help yourself to a drink, darling.'

'I love salad,' Jochen said, reaching for a can of Coca-Cola. 'I like cold food best.'

'Good boy.' My mother drew me aside. 'I'm afraid he can't stay this afternoon. I can't manage with the wheelchair and whatnot.'

I concealed my disappointment and my selfish irritation – Saturday afternoons on my own, while Jochen spent half the day at Middle Ashton, had become precious to me. My mother walked to the window and shaded her eyes to peer out. Her kitchen/dining-room looked over her garden and her garden backed on to the meadow that was cut very haphazardly, sometimes with a gap of two or three years, and as a result was full of wild flowers and myriad types of grass and weed. And beyond the meadow was the wood, called Witch Wood for some forgotten reason – ancient woodland of oak, beech and chestnut, all the elms gone, or going, of course. There was something very odd happening here, I said to myself: something beyond my mother's usual whims and cultivated eccentricities. I went up to her and placed my hand reassuringly on her shoulder.

'Is everything all right, old thing?'

'Mmm. It was just a fall. A shock to the system, as they say. I should be fine again in a week or two.'

'There's nothing else, is there? You would tell me…'

She turned her handsome face on me and gave me her famous candid stare, the pale blue eyes wide – I knew it well. But I could face it out, now, these days, after everything I'd been through myself: I wasn't so cowed by it anymore.

'What else could it be, my darling? Senile dementia?'

All the same, she asked me to wheel her in her wheelchair through the village to the post office to buy a needless pint of milk and pick up a newspaper. She talked at some length about her bad back to Mrs Cumber, the postmistress, and made me stop on the return journey to converse over a drystone wall with Percy Fleet, the young local builder, and his long-term girlfriend (Melinda? Melissa?) as they waited for their barbecue to heat up – a brick edifice with a chimney set proudly on the paving in front of their new conservatory. They commiserated: a fall was the worst thing. Melinda recalled an old stroke-ridden uncle who'd been shaken up for weeks after he'd slipped in the bathroom.

'I want one of those, Percy,' my mother said, pointing at the conservatory, 'very fine.'

'Free estimates, Mrs Gilmartin.'

'How was your aunt? Did she enjoy herself?'

'My mother-in-law,' Percy corrected.

'Ah yes, of course. It was your mother-in-law.'

We said our goodbyes and I pushed her wearily on over the uneven surface of the lane, feeling a growing itch of anger at being asked to take part in this pantomime. She was always commenting on comings and goings too, as if she were checking on people, clocking them on and clocking them off like some obsessive foreman checking on his work-force – she'd done this as long as I could remember. I told myself to be calm: we would have lunch, I would take Jochen back to the flat, he could play in the garden, we could go for a walk in the University Parks…

'You mustn't be angry with me, Ruth,' she said, glancing back at me over her shoulder.

I stopped pushing and took out and lit a cigarette. 'I'm not angry.'

'Oh, yes you are. Just let me see how I cope. Perhaps next Saturday I'll be fine.'

When we came in Jochen said darkly, after a minute, 'You can get cancer from cigarettes, you know.' I snapped at him and we ate our lunch in a rather tense mood of long silences broken by bright banal observations about the village on my mother's part. She persuaded me to have a glass of wine and I began to relax. I helped her wash up and stood drying the dishes beside her as she rinsed the glasses in hot water. Water-daughter, daughter-water, sought her daughter in the water, I rhymed to myself, suddenly glad it was the weekend, with no teaching, no tutees and thinking it was maybe not such a bad thing to be spending some time alone with my son. Then my mother said something.