This was growing where it should have been: at the foot of a wall. On first sight you might think it's a funny member of the buttercup family because of the colour and size of the flower. But closer inspection soon dispels this notion: there is an odd protruding long narrow ovary and no sepals; break the stem and a yellow/orange latex is exuded. This flower has lost its anthers so looks slightly more peculiar it would have done, when it would have even more closely resembled a buttercup. It actually has more in common with horned poppy and that's because it is a cousin and member of the Papaveraceae.
Its common name reinforces the false connection to the buttercups as Lesser Celandine is in that family. The Latin binomial for Greater Celandine is Chelidonium majus which derives from the Ancient Greek for swallow (the bird). The connection with swallows has variably been attributed to its habit of coming into flower when they arrive and ceasing when they leave, but a more imaginative explanation was that adult swallows used the orangey/yellow latex to cure blindness of their young before migration. (Sometimes it's hard not to think of our forebears as utterly mad!)
It is a potent medicinal herb and has a long list of uses. The latex has long been used to cure warts, leading to a plethora of vernacular names such as wartweed, wartwort and wartflower. The herb and root are analgesic and sedative. It is thought to have been introduced and its habitat preferences in this country suggest a close link to human habitation and propagation.