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The door to the conservatory only ever had one key. When that slipped out of sight one day, the door remained unlocked until the day the insurance renewal came up. "Do all your outside doors have working, five-lever locks?" she asked. I put the phone down reluctant, this once, to admit that my door is always open.

Not that there's anything in the conservatory. It's not even a conservatory, it's more of a small lean-to made from plastic panels and glass. Shabby as it is, it faces south and warms up as soon as the early morning sun climbs above the pyracantha bush that marks the edge of our territory in this crowded town. The plants like the warmth and clothes dry quickly when it rains outside, drippping on to the stone-tiled floor.

I keep it warm in winter with a 10-year-old electric fan heater. There are some house plants that don't like the cold, a cat-flap: an ancient relic of the previous owner. Apart from an old, fraying mat to wipe shoes on, the only other thing in the room is a doorway into the outdoor toilet. That has damp seeping up from the floor ready to crumble the pale blue paint. There's nothing of value there; nothing to steal. But still, who wants to have unwelcome strangers walk in through a door that can't be locked? Even when the room contains only a few fragile plants and an outside toilet.

There's no doubt what insurance companies think about leaving an outside door unlocked. They'd wriggle out of any claim faster than you can say 'caveat emptor'

Taking a uPVC door apart is not as difficult as it looks. All you need is a powered screwdriver, the right bit and a charged battery. Each press of the trigger brings another screw spinning out of its home. The drill whines and the bit bites into the brass screwhead, dragging the screw out of its quiet burrow. Do that ten times for ten little self-tapping screws that hold a long, stainless steel strip to the door, and you are almost done. This door, like most plastic doors, has the main lock at hand height and secondary bolts at the top and bottom of the door. When you flip the door handle upwards, those bolts shoot out engaging with slots in the door frame to keep the insurance companies satisfied. But only so long as the thing can be locked shut.

Spin eight more little grub screws out, one by one, and the strip comes away from the locks and bolts. Put the screws down on the workbench, in a cluster next to the self-tappers so that they are ready to go back home. With no screws left to hold it in place, the strip comes away, waving drunkenly in its new-found freedom. Water gushes out as the seal is released. Rain from a week ago, so the door must have a leaking seal somewhere. That probably means the mechanism inside has rusted. Make a mental note to oil everything and check for holes and for rust. Once the water has drained, the two secondary bolts come free and then the main lock. The strip is shiny, bendy and unwieldy to handle. Put it down, out of the way to avoid damage from dropped tools, or clumsy footwork.

Behind a series of protective strips, there is a small hole that hides one critical screw near the lock barrel. When that screw is in place, the lock is secure. Remove the screw and the lock can come out of its chamber like a spider comes out of its hole. This vital screw hides deep within the lock, like a secret that has to be teased out. Its hole is hidden behind the sliding steel strip that connects the lock to the top and bottom bolts. Push the door handle down, and a hole on the strip lines up with the hole in the lock, revealing a deeper hole. At the bottom of that dark, near-inaccessible hole is the one vital screw that keeps the lock in place.

This is no ordinary screw. You need a torch to see any part of it, but its head is neither a slot nor a cross. It is a hexagon and you need the right size of hex wrench to remove it. I have the whole set of drill bits in hexagon sizes, but each of them is only a few millimetres long. This screw is buried 20mm deep, behind a shutter and in an awkward hole right at the heart of the lock. None of my ordinary hex bits can get close, even in the hand-held driver. I have to find my old set of original hex wrenches to coax this shy little screw out of its hiding place.

Down into the cellar to get my toolbox. I love going into the cellar, especially on hot days. It was built as an underground coal cellar, so it keeps an even temperature all year. On a hot day it is blissfully cool. My house was built about 100 years ago, and the walls reach down into hard, white chalk. The same rock that makes Dover’s cliffs white. A few minutes delving in drawers and toolboxes brings forth the keys.

Despite being plain, matt black, they are beautiful. There is no better example of Louis Sullivan’s dictum A hex wrench is at once the most simple and the most sophisticated of tools.

The outward apearance is utterly simple: a bent bit of wire. Each key made from a slghtly fatter piece of wire. But what wire! Each wire has been carefully shaped and precisely conditioned for its purpose.

A hex wrench is made from a length of black, tempered steel and comes in the shape of an uppercase L, with one long arm and one short. The profile is a perfect hexagon with no embellishment or artifice. It fits perfectly in the hexagonal recess on a screw. Each wrench is so perfect and so hard and fits the screw so closely that the corners are able to twist any screw from its home. You can apply more torque through a properly-fitting hex wrench than through any other type of screwdriver*.

* Stupotsays the 'torx' wrenches offer a little more than the hex wrench. And he's right.

The conservatory is hot. Although I am doing this in early September, the day is unseasonably warm. The sun is high over the orange berries of the pyracantha and shining straight onto the roof, down into the glass-enclosed space within. A bead of sweat drips from my brow.

The hex wrench slips home with a perfect fit. When you put that wrench in, you automatically give it a little wiggle, just to see how much slack there is. It's not a conscious thing, but every person who ever used a hex wrench does it. With this screw, there is no slack at all. One quick twist, a slight bend on the wrench and the screw starts moving. Twenty more turns and the screw is out.

At this stage, you should be able to put the key into the lock, turn it through a small angle and then wiggle the barrel out of its housing. There was only ever one key to this lock, and that was sitting in its own private key heaven. I had to find some other way to get the barrel out.

The heart of a door lock is a cylindrical barrel that can turn within its enclosing metal block. A line of five or six small holes run from the edge of the block towards the centre of the barrel, across the line that separates the barrel and the block. More of these holes means a more secure lock. In each of these holes lie two metal pins and a spring. These pins are different in length and pushed into the barrel by springs. With no key in the lock, the pins lie across the join, preventing the barrel from turning in the lock.

When the true key slides into the lock, the pins line up: a short pin matching a small notch on the key; long pins matching deep notches on the key. The lengths of the pins match the shape of the key and when the pins all line up, the barrel is free to turn. If the key is the wrong shape, then the pins don't line up, and some of them wedge between the barrel and the lock, preventing the thing from turning. It's simple, but very effective.

If you don't have a key, you have two or three alternatives. You can bring your lock-picking skills into effect, messing about with tweezers and paper clips and trying to make those tiny brass pins line up.

Or you can get a power drill with a large drill bit and cut right into the barrel of the lock, mashing the pins to oblivion on the way. A week earlier I had bought just the right drill bit. Tungsten carbide coated for extra cutting power.

Lock makers all know about drilling out the lock, so they make it hard. They put case-hardened, tungsten carbide shields over the barrel. A drillbit just skitters over those shields. I was lucky, this was an old lock, and cheap, to boot, so the drill bit into the metal barrel with a sweet, steady cutting action. Small shavings of metal fell away from the barrel. Push harder on the drill and the flakes of metal get bigger, while the bit bites harder and deeper. Those pins, buried inside the lock are made from something a lot harder than the body material.

You set the drill running, and push the bit steadily deeper into the lock, until grawnnchh! The bit has hit one of those pins and stopped. The bit can't cut into the pin, so it has to stop. The motor is still turning, so the chuck turns around on the stationary drillbit. The sound is not good and you release the trigger immediately. Reverse the drill a fraction, withdraw the bit and look. Reverse the drill once more, and push the drill back in, only pushing very gently, to clear away the ragged edges, until the bit stops one more. Then go in with a fine screwdriver and remove the loose pin. And the second pin behind it. And the spring. Continue drilling.

An hour later, sun shining, twelve pins on the floor, the pile of metal shavings growing ever higher. Suddenly the barrel starts to spin on the drillbit, and you know that you have drilled the lock.

It feels good. Spin the barrel around on the drill once or twice for fun, then go in more carefully and try to line everything up until the barrel can slide through the lock.

It works! The barrel comes free.

It takes no time to remove the old barrel, replace it with a new one, and then put that vital screw back into the heart of the new lock. A final twist with the hex wrench and all is well. Test the mechanism, operate the key, see the bolt shoot out, check the handle still works. It's all good. Put the spare keys somewhere safe.

Another few minutes to put some oil on all the moving parts, align the strip, and fasten it with the same old screws.

The door closes, and you hear that satisfying snick as the bolts snap home. Oiled, quiet, secure. Screws and bolts tight in their homes.

Clear away the tools, sweep up the metal shavings, sling the old, drilled barrel in the trash. Lock the door.

Now that feels good!

There is one other door to the conservatory. That has only one key, and I do have a spare lock. Next weekend, maybe.

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