Calamintha nepeta 'Blue Cloud':
Spikes of small, soft blue flowers above mounds of minty scented green leaves. Loved by bees and butterflies. Hardy. Up to 30cm.
Campanula lactiflora (Milky bellflower):
Clusters of blue flowers on tall stems in mid-summer. Ideal for filling out borders and adding height. Fills out quite quickly too. To 1.5m. Hardy.