Anagnalis tenalla. (Bog Pimpernell.)
This is a hardy perennial thatcreeps along the ground creating a carpet effect of patches a few cm across. The flowers are small but subtly beautiful and cover the carpet on simple golden stems in spring.
These are tradtionally grown as marginals and like to be near water. I have known them to grow elsewhere even in dry stone walls. However they will go dormant as nothing but roots if allowed to dry out.
Easy to grow comes in a 9cm pot