Aster x frikartii 'Monch':
Soft lavender blue flowers in late summer. Very early flowering and shorter than many asters at about 80cm. Easy and reliable.
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.
Cichorium intybus (Wild chicory):
Large cup shaped white flowers over soft ferny foliage. Thrives in sunny, well drained spots. To 2m
Corydalis elata:
Summer flowering Corydalis with bright blue flowers above soft green foliage. Excellent in some shade in moist soil. Hardy. Up to 30cm.