"The leaves and thick paper I had to hand didn't work, even when I held them on top of the rod. The paper almost went, but not quite." -- Paper can sometimes be made to work if you scrape thin bits off the surface with a knife. A spark from a flint has a high temperature but low heat. It's hot but it cools down quickly, so it works best to ignite something that's very finely divided. It can't heat thick paper enough but it can heat thin scrapings or paper dust. Finely divided tinder is also more flammable because it has more exposure to oxygen. These are reasons why cotton wool is so good.

I find commercial tinder works best, and carry some of the SparkLite tinder, but like a cigarette lighter or matches it will eventually be all used up. If you can improvise tinder, a flint rod will start fires indefinitely. There are plants that work, but in an urban setting paper may be easier to find.