Geranium 'Patricia', I love the magenta zing of these flowers which come from June for a couple of month. The plant makes a nice rounded low clump which contrasts nicely with the spikier plants. I suggest three or four of these repeated at intervals so your eye is drawn down the border. For middle of the border
Penstemon 'Andenken an Friedrich Hahn' - Elegant spikes of small, tubular, foxglove-like flowers appear from July to October among lance-shaped, bright green leaves. This vibrant, easy-to-grow perennial commemorates an 18th century German astronomer. It will quickly form large, leafy clumps and is perfect for adding a splash of colour to the middle of a sunny, well-drained border.