I have a catmint trailing over a wall and earlier in the summer, underneath in the path, were masses of tiny seedlings which looked like catmint. I potted a lot of them on and they grew up tall and straight with serated edged leaves. I then noticed a similar looking plant in the same border as the catmint which was in flower with yellow flowers. I have never seen this plant before and how it came to self seed itself beneath the catmint is a mystery to me and now its growing in the border. Please help, does anyone know what an earth this mystery plant could be? Its about 0.5m tall with green serated leaves and rounded yellow flowers about the size of a 1p.