I Googled "Campanula" under images, and found Campanula alliarifolia: http://theseedsite.co.uk/profile104.html
The article says:
"Campanula alliarifolia is unlike many other Bellflowers. It is a coarse, leafy, almost bushy, plant with rough stems and crinkled heart-shaped leaves on strong arching stems. Throughout the summer and into autumn, it bears clusters of elegant white tubular flowers about 3" long at every leaf-joint, the buds opening throughout the season rather than from bottom to top."
Wikipedia gives some more information http://en.wikipedia.org/wiki/Campanula_alliariifolia
Is that the same as the plant you found?
The Keble Martin flora says it's a garden escape, mostly found in Devon and Cornwall, but it could well have spread since then, as my edition is from 1978.
your plant is the Giant Bellflower Campanula latifolia which is a native to this country. It is absent from the south where it is replaced by the rather similar but smaller Nettle-leaved Bellflower C. trachelium. It often shows these rather pale coloured flowers.
C. alliarifolia differs in its much more heart shaped foliage and is a shorter, less upright plant - its flowers are more bell shaped and smaller and an Ivory-white.