I am looking at a bed bug. Two bed bugs, actually. And two surprises: first, they are not minuscule, but about half a centimetre long, rounded and reddish brown and easy to see. And second surprise: they are not in a bed, but on a living room table, nestled into a corner underneath, trying to escape from my investigative pen.

The small, tidy flat's distraught inhabitant, a Somali woman, is in the corridor outside. She hadn't cleared the room as instructed, she had forgotten the pest controllers were coming, and she was in bed (with her bed bugs) when we knocked.

Uriel Bowen has been kind. He has agreed to treat the room anyway, but that means stripping her bed, clearing away the personal possessions that might get damaged by the chemicals and ordering her to leave for two hours. Having been instructed not to rub up against anything on the way in, in case I pick up “visitors”, I am none too keen to muck in with the bed-clearance. That said, it isn't easy anyway, in protective suit and rubber gloves and with a gas mask dangling from my wrist.

There probably isn't a less sexy job in the world than being a pest controller. And possibly no worse a place to be one than in Central London. “It's nice to do something slightly out of the norm,” Mr Bowen said. “There is money in muck - for the reason that for most people it's the one thing that they want to stay well away from.”

Going on dates, he admits, can be tricky. “When you're in this sort of business, it's not necessarily a talking point. When you start talking about it, the first impression that women usually get is ‘urgh'. It's not an occupation you brag about.”

Mr Bowen's work takes him down into the depths of human detritus. We started in the service tunnels under Leicester Square, laying packets of bait for rats amid the snaking cabling and fat pipework that twists under Central London, the gas and electricity and the fibre optics.

Deeper and deeper we went, leaving behind the occasional shafts of natural light from the pavement grille covers above. The traffic rumbled overhead, water trickled somewhere below us - yet it is strangely peaceful down in the damp warmth of the city bowels: all the noises are muted and, best of all, mobile phones don't work.

The rats take the bait but, oddly, the pest controllers rarely see a dead one: somebody one day might like to work out where rats go to die. We tramp down, round, up through another tunnel, a long narrow set of steps, and back into the dim light of Leicester Square in autumn.

Tourists and commuters step around us, uncurious, as we all are when men in fluorescent jackets and hard hats hover around holes in the ground.

We head off for a housing estate farther north. Rats have been seen and Mr Bowen and his colleague Andy must raise the manhole covers to bait the sewers. This time the smell is horrid. A pile of - better not to think about it - squats heavily in the base of the trickling sewer five feet below. They tie a fat biscuit of poison on a long wire and dangle it down on to a ledge, secure it, cover the hole, leave.

On the way to the bed bugs, Mr Bowen gives me a bit of natural history. The bugs can live in light fittings, electric sockets, in the cardboard tubes on metal hangers, and can survive without food for up to a year. “They can get in absolutely anywhere.” The egg cases, often laid in mattress seams, are completely resistant to chemical treatment.

This is the lady's second visit from the pest controllers and while she insists she hasn't seen any bugs for three weeks, she turns over her living room table and there they are. “It's so horrible,” she moans. “I don't know how it come. Every time I clean the house. . .” She shows us other hiding places where they have been found in the past: behind the hook on her wall, in the picture frames. My arm starts to feel itchy.

Mr Bowen searches the slats of the bed and finds “spottings”, drops of digested and excreted human blood - “that's their toilet, basically”. First he sprays along the mattress, bed, table, sofa, etc and then we don gas masks for the “fogger”, which fills the room with a fine mist of pyrethrin, “one of the strongest poisons known to man”. My gas mask puffs in and out with my breath. My hands sweat in the rubber gloves. I feel claustrophobic and am pleased to leave. The tenant, hovering outside, apologises for not making us a cup of tea.

Another flat in the same block has reported mice. Mr Bowen remembers the place and as soon as we enter the flat, I can understand why.

The first thing you notice is the smell: stale, warm, rank. Pieces of paper are shredded all over the floor. The sitting room is piled high with old Guardian newspapers, books about Marx and Nietzsche, Joyce's Ulysses, Gombrich, and pills and hospital appointment letters. In the midst, in a leather armchair, picked out by a shaft of sunlight, sits an old man, nursing a cup of tea. Dirty clothes are piled in the bedroom, a large yellowing spillage congeals beneath the fridge. The man says he thinks the problem is coming from other flats; anyway he is going on holiday in three weeks, he tells us, his last two were cancelled as two of his siblings died.

“I'm sorry to hear that,” Mr Bowen replies kindly. He rebaits the mouse traps, makes a note to tell the estate office to plug the hole behind the toilet.

The worst scene the pest control officer ever saw was after a family reported rats and a smell coming from an old man's flat. The police couldn't get through the door so called the fire brigade who found a body in the front room of a flat piled top to bottom with rubbish, which is why the door wouldn't open. “The tenant had created tunnels through the rubbish. We crawled in, went through the tunnel; it would break off - front room, bathroom. The bath was his bed.” A police officer vomited.

We shut the door on this old man and his Nietzsche and his dreams of a holiday. “It would be wrong to pass judgment,” Mr Bowen cautioned. “This is his home. Provided it doesn't affect other people, we can't dictate to people how to live their lives.”

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