Thanks for the responses. There are loads of them, though I've never thrown out any seeds or anything. I'll pull them all up, hoping they haven't already spread.
It's odd though, as over the last couple of years I've had a mix of things spring up out of nowhere. A fig tree started growing. Then two blackberry brambles. This year a rose bush started growing and is doing well, though sadly in a somewhat less than optimal spot with very little soil around it. Ironically, for the last two years, I've been buying loads of seeds and bulbs and had very little success. These random things thrived!
Well at least they did until I got rid of them. The blackberries were nice but were taking over one end of my small garden and probably took too much out of the soil for the flowers I want to grow. And I saw the fig tree as a potential long-term threat to surrounding concrete and fences, so I'm trying to kill it off too. Another tree has appeared directly behind a fence (haven't identified it yet), so I have to speak to those neighbours before it gets big and cracks the floor.
And now I have an ant colony being built rapidly in a makeshift compost bin!