Calochortus aureus is what springs to mind.

Sisyrinchium brachypus.

None of my Sisrinchium have the colouration of this at the base of the petals. Also the pollen is orange rather than white as here.

Thank you both. I had considered and discounted both suggestions because the calochortus has only three petals and the sisyrinchiums do not have the distinctive marking on the petals. I would welcome more suggestions.

Some indication of scale would be useful.

Try Homeria

Thank you Owdboggy. I think you are correct but I'm not sure whether this is a hybrid or one of the species.

I found the source of my plant in my seed sowing book. I sowed seeds of Sysirinchium macrocarpon from the SRGC seed exchange on 1.3.14. I have just checked it out on this website .
http://www.plant-world-seeds.com/store/flower_seed_categories/SISYRINCHIUM_SEEDS
Thank you both for all your help.
Owdboggy it is about 5 inches high and the flowers are over an inch wide so a very big flower on such a tiny plant.

I would go along with that. Did you spot its common name, Scotsgran, 'Blue-eyed Grass'! Nothing blue about this one but it is a name generally given to all the Sisyrinchiums.

Yes Bb, I did spot the 'blue-eyed grass' name. That is a very good reason for knowing the latin name of a plant you want. Did you also read that this is one of the nicest yellow Sisyrinchiums available. Thank goodness for the SRGC SEEDEX. I doubt if I would have heard of it anywhere else.