This grower is sure of this plant identification.
Danesblood bellflower, Clustered bellflower, Bellflower
Campanula glomerata (Danesblood bellflower). This is a vigorous clump-forming perennial which is native to Europe and Asia with bluish purple, whitish green flowers which are held in dense clusters in the summer. These plants will grow to be about 2-3ft tall.
You should remove the faded blooms to encourage further flowering. You should grow them in full sun with well drained, moderately rich soil. This plant will need regular dividing as it is a very rampant grower.
Walls, rock garden, border, naturaliser.