@johnjohnston Wikipedia mentions it was introduced to Europe in the 17th century. There's a map showing distribution here:
As the name implies, it blossoms at dusk in Utah where I've encountered it most frequently. But the blossoms last through the night and into the next day for a while.
@oroboros @jeremycherfas Thanks, I don't think Oenothera, they seem to North American, I found this quite far from cultivation so don't think it was an introduce or escapee. After a few twitter suggestions I now think it might be Wood Sorrel, quite a few miles away from the nearest woodland, and a bit higher than expected.