The five-minute memoir: Rupert Christiansen recalls an intruder in Clapham

One joy of the first-floor flat in Clapham in which Ellis and I live is a French window, opening from our bedroom on to a terrace and overlooking a verdant panorama of back gardens which less than 150 years ago would have been densely wooded.

The water table is high here, and everything grows like Topsy: this is ground over which wild life still secretly asserts primal rights, and no amount of mowing and pruning can altogether tame it into clipped suburban neatness.

On hot summer nights, we keep this French window open, protecting ourselves by pulling shut a retractable security gate: the gap between its concertina steel bars is barely four inches, and no fauna except pesky insects have ever managed to penetrate them. Or so I fondly imagined.

Sometimes in the small hours I wake and hear through the curtain what I assume is a neighbour's cat padding around on the decking, but whatever it is keeps a respectful distance and the noise never seems threatening. Yet I was baffled and faintly alarmed one Friday morning to find a raw sausage, unmarked by teeth, lying on the carpet by our bed, apparently deposited through the bars.

Ellis and I briefly debated its significance before realising that we would have to laugh its arrival off as one of life's little mysteries. That morning the temperature dropped and rain began to fall, so for the rest of the weekend the French window remained firmly shut.

At about 3am on Monday, Ellis woke with a start and shook me. "There's something in the room!" he said. "Don't be ridiculous," I said grumpily, "you've just had a bad dream."

He dropped back to sleep, but as I lay awake I heard creaking. Very odd, I thought: did something need oiling? Then I drifted off, and next morning we pottered about and went to work, thinking no more about these minor disturbances.

Four hours later, I answered a telephone call from my downstairs neighbour, Daphne. She sounded hysterical, not surprisingly, and this is what she told me.

Our burglar alarm had started ringing mid-morning, so she had gone up to investigate. She switched the alarm off and the flat seemed innocently empty. But when she peered into our bedroom, she spotted two furry peaked ears peering out from inside one of the wooden storage drawers under the bed.

When she screamed, the fully grown owner of those ears leapt out of the drawer with a yelp and began running hysterically round the flat. With heroic presence of mind, Daphne grabbed a broom from the kitchen and found the keys to open the French window. Then began a pitched battle, in the course of which Daphne tried to poke the fox out of the sitting room, kitchen, and bathroom back out through the French window into the urban wild whence it had come.

Foxes face one out; they don't have an instinct to flee when fixed by human gaze, their view being that they settled here first and that we are the interlopers. But its fear was none the less greater than hers, and later I would find fresh warm turds in four different places round the flat, presumably dropped in scampering panic.

It took Daphne 15 tense minutes to complete the manoeuvre, and several hours to recover her composure. We returned home that evening with grateful flowers and attempted to figure out the course of events.

Given that the French window had been closed since Friday morning, the fox must have been in the flat for at least three nights, having slithered between the gate's bars without us hearing and settled on top of the mass of old theatre programmes that I keep in the drawer.

It – he, she? – left little trace: only a tiny puddle of odourless urine on the carpet under the bed and a few desiccated turd pellets in a corner of the drawer. Rather than feeling the rage that normally boils up at the invasion of the uninvited – whether mice, junk mail or burglars – I came to pity a desperate creature which neither ate nor took anything.

My conclusion is that it must have been wounded or sickening and in need of sleep and shelter, away perhaps from a lair noisome with bossy spouse and hungry jostling children. Foxes prowl the street like nocturnal traffic wardens hereabouts, but I feel that it just couldn't take any more of the hunt. Was that raw, unchewed sausage a thoughtful offering, advance rent? If so, I am rather touched.

Rupert Christiansen's 'I Know You're Going to be Happy' is published by Short Books at £12.99