Quaking Grass (Briza maxima)
Or as we called it smaller relation as kids "Dothering grass". The internet doesn't seem to have heard of dothering so I assume it is yet another Cumbria word I grew up with. Dothering basically means shaking, as the seed heads/flowers do when the wind blows.
Clickychick bought the original from an ex colleague of mine at an open garden event. She was attracted by the way it was growing in their gravel drive. It seeded into our gravel path and unlike the drive it came up as a thick patch rather than the single plants she had seen in the drive. Now it has been transplanted into parts of the garden where it is welcome to do its own thing. Which probably means it is going to decide to disappear next year.