Mystery solved!

The mystery plant has been identified by Lynne. It's a Pseudofumaria lutea which is known as the Rock Fumewort or Yellow Corydalis. Fumewort, it seems, is a common name which can mean any of a dozen different species. The obsolete Latin name for this plant is Corydalis lutea though I have no idea how a plant name becomes obsolete.

Many thanks to everyone who made a suggestion and to Lynne for finding the little rascal.