Thank you to Sue of Mousenotebook who has identified my mystery plant/weed as Wood Avens.
Wikipedia states: Geum urbanum, also known as Wood Avens, Herb Bennet, Colewort and St. Benedict’s herb,is a perennial plant in the Rose family (Rosaceae)which grows in shady places (such as woodland edges and near hedgerows) in Europe and the Middle East.
Usually reaching a height between 20 and 60 cm, wood avens blooms between May and August and its flowers are 1 – 2 cm in diameter, having five bright yellow petals. The hermaphrodite flowers are scented and pollinated by bees. The fruits have burrs, which are used for dispersal by getting caught in the fur of rabbits and other animals. The root is used as a spice in soups and also for flavouring ale.
In folklore Wood Avens is credited with the power to drive away evil spirits, and to protect against rabid dogs and venomous snakes. It was associated with Christianity because its leaves grew in threes and its petals in fives (reminiscent of, respectively, the Holy Trinity and the Five Wounds). Astrologically, it was said to be ruled by Jupiter.
In herbal medicine
Sue warns me that if her identification is correct then I am in for trouble. So it is likely to be a head-to-head with the pernnial geranium (cranesbill). Watch this space, it is likely to disappear under a mound of green leaves.