Yellow Pimpernel is yellow, small and quite similar to Creeping Jenny. This is Yellow Pimpernel in the photo below.
This was photographed on the edge of the car park just up from Bigswier in the Wye valley ( English side). A nice little car park with several other woodland flowers like Primrose, Wood anemone, Barren Strawberry and a few Bluebells…. so very attractive in Spring time.
In the same genus is Creeping Jenny (Lysimachia nummularia ) which has flowers that are more cup shaped, but still with 5 petals and the same shade of yellow. It is more of a ‘garden’ plant. and prefers wetter habitats. Its leaves are a little different, being more similar to Violets ie heart shaped. At present I do not have a photo of one… I will have to take a trip to my local garden center, so here is one with thanks to wikipedia.
Here is some Yellow Pimpernel which is not in flower but shows its leaf shape quite well… rather like a robust chickweed.
Flowering time is normally May to June.
You can see in the close up photo above that it has five stamens and there is a centrally located stigma, but it is difficult to make out in this photo.
William Morris produced a design known as Yellow Pimpernel, which has some rather simplified depictions of a small yellow flower looking vaguely like a Yellow Pimpernel, along with some much bigger flowers that look like Peonies to me. Here is a cushion that was for sale on E- Bay with this design.