A mystery solved.  Last winter a tiny seedling was found in the compost of a new amaryllis.  After tending it carefully it flowered when we were away last week and have discovered that it is Safflower (Carthamus tinctorius).   It has many long sharp spines on the leaves with flowers that resemble a thistle and grown commercially for the oil that can be extracted from the seeds.  The oil is used in foods and in a variety of other products and the flowers are sometimes used as a food dye or as a substitute for expensive saffron.  Commercially it is grown in USA but I don’t think it is ever grown in Britain and wonder how it got into the amaryllis compost.