Hi Peter
A good question and not one a have an answer too, it's not at all common around Bewdley or other parts of the Wyre Forest In fact I have only seen a small introduced clump in Kidderminster but it is an attractive plant that likes damp ground and can be found for sale in garden centers, I bought some for my garden and does do well but may need to be thinned out as it can spread quite happily if you let it, and it's also a good food plant for bees! But as to how common it is growing around the country I'm not sure though I know it grows well up in Northern England and is often introduced around farms and villages!
Jez