It's 3.30am. The last guests left about an hour ago, but I'm still awake, my mind buzzing with a million little thoughts and no small amount of caffeine, it being rude to appear sleepy and inattentive at one's own leaving party. I think one of the best things one can say about a goodbye party is that way more people turn up than you expect. (And yes, you lot, I'm blogging about it just after you leave. What did you expect?)

Wow. It's beginning to sink in now: I'm seeing lots of people for the last time I'll see them for years and years. It's my last weekend I'm actually going to spend in London (next weekend is elsewhere in the UK for my brother's wedding). The finalities are beginning to stack up. The goodbyes are getting harder and harder. I'm really leaving London behind.

What's London to me, anyway? When I think of London I always think of the first place and time I ever lived in London: Clapham South, back in January of 2000, when it was still a rather down-at-heel and studenty place to live, and the local shop was a Europa Foods instead of the M&S that's there now, and the hospital was an abandoned hulk instead of brand-new des-res conversion flats with views of the park. Three-storey Georgian buildings, dimly lit and silhouetted against the great orange glow of the city.

I remember when that was novel. The idea that a city could be so big and so bright that it lit up the cloudy sky. Not an urban jungle, more an urban hive. Brooding and grim but also dark and dangerous and sophisticated and exciting. A great big machine made of metal and concrete and plastic and neon, full of hidden nooks and crannies. A boy who always tried every lever and pressed every button on principle, presented with a huge toy with more buttons and levers than I could ever hope to find, far less push and pull.

Dark is definitely the word. London is a wonderful place in summer but at heart the city is a stony grey, perhaps a little cold but solid and massive and reassuringly durable. This is a city that has been almost continuously occupied for almost 2000 years, possibly more than 3500, an almost unimaginably long time that encompasses the rise and fall of not just empires but whole eras of history, ways of thinking, cultures and societies and fundamental levels of technology. You can walk today down streets that were already ancient when they featured in the great fire of London, an event itself more than 100 years older than the entire nation that I am about to become a part of.

London has also been for me the strange, weird, magical land that took my lifelong passion for making pretty things appear on the screen and said "hey, we'll pay you to do that." Objectively I know that few people are as interested as I am in what I do, that it serves a commercially useful purpose, that creating order from disorder creates value. But at the back of mind I'm always amazed that I managed to trick an entire city full of otherwise sensible people into letting me turn that into a career, and by extension a life. I fool the people who give me jobs, they give me money to fool other people who give me a place to live, and they give me so much money I have money left over to buy food and drink and electricity and books from gloriously gigantic bookshops that are just sitting there, huge piles of crisp, untouched knowledge. I keep thinking someone going to notice, and say "Hey! He's not really an adult! He's just some kid who does web pages! That's not a proper job! Take his house away, send him back!"

And I suppose that's part of the anxiety of moving to a whole new country. I fooled one city. I know my tricks work here. Can I really pull that trick off again? And what about all the people? I met people -- lots of them at that other little island of magic, university -- and fooled them into thinking I was witty and clever, so cleverly that they agreed to hang out with me, invited me to go places and do things with them. What if the new city sees through me?

But then I remember the important lesson I learned when I was still hiding behind my Seldo-mask, which is that I am nowhere near as clever as I think I am. Certainly not clever enough to fool all those people. So despite my incredulity, I must really be what they think I am, and whatever that is, they think it's worth paying to play with code and inviting out to dinner. Something to cling to on my first night there, I think.

And behind me I leave something I've still not adequately defined, I as always being a stickler for getting the right word for the thing. London, not an artifact, maybe not quite a machine, but a system, a culture. Ah, there's that word. A collection of experiences and customs and tradition and memories and laws and weather and buildings and people and events and organizations all intertwined into a single amorphous unit, a sum far greater than its parts for which I can feel nothing but deepest, abiding love. A culture that has embraced me and raised me and changed me for the better, a culture I hope I have been able to give back to.

I am off to try a different flavour of culture, but as I say my final farewells I know I'll never really leave London behind. London as it moulded me has left its fingerprints all over me, its gritty grey stone permanently under my skin.

Thank you for everything, my first and greatest city. I'll be back someday.