What Daisy is this?

My mum has two of these plants growing on her wall for many years. They're huge and don't seem to need much care if any. As you can see from this photo taken in the summer, the bees are mad for it. She has tried taking cuttings but so far they have all failed. I harvested a big bag full of seeds from it, but we would like to know exactly what it is before we start sowing them so we can do things the right way as much as possible.