I just discovered what the plant was yesterday when I finally decided to find out if it was comfrey. As soon as the flowers go away or we get to late spring when there are lots of other flowers about (for the bees' sake), I'll be snipping the stem to prevent seeds forming and chucking that in the council bin.
I am now also going to PREY upon the plants for their leaves to compost. I read elsewhere that they are rich in nitrogen- similar to comfrey, which makes sense. Like comfrey, the deep tap root can extract nutrients from far down. From now on, the green alkanet will be my compost supply of green leaves- if it wants a place in my garden, let's see if it can afford the cost in leaves.