Looking through the Biological Records in Essex website I see that there are "about 5 sites still extant, 'gardened at 2 sites'". I have a population in my garden (CO4 3UP) (TM0301225378), which is previously unrecorded.
They were growing here when we moved into the house 25 years ago, and have seeded themselves around ever since. When we moved in the house was only 3 years old and the former owners had done little to the garden. Formally the site had been agricultural land, but I don't know of what type. But it certainly seems that the plants are of wild stock.
If anyone has any knowledge of any other nearby sites I would be interested to know. My email address is j_hartwell@sky.com.