Here is a better picture of the mystery plant from yesterdays six on Saturday. It was meant to be an Echinacea but it clearly is not.
I will keep updating this page as it develops.
Update 22nd May
I have a new idea: maybe it is Saponaria ocymoides.
It is very close, including the redish stems, the pink 5 petalled flowers, the tube beneath the flower, the leaves are opposite and the general leaf shape. Some pictures of it have thinner petals than others.
Update 21st May
A number of flowers are out now.
Update 20th May
The flowers are about twice the size of my forget-me-nots and Brunnera and are looking less like forget-me-nots now. Another picture to follow when a few more flowers have opened.
Whatever it is it is quite pretty and almost glows in the twilight.
Update 19th May
A flower opened. It is pink and looks very much like a forget-me-not in size, number of petals and general shape.
So congratulations to the anonymous commenter who said that it looked like a forget-me-not in the original post.
It is not the same kind of forget-me-not that I have all around the garden, but it could be some other forget-me-not species or something closely related that has simlar flowers.
First picture 13th May