Looks like a campanula but I don't know which one.

I'd agree a white Campanula but would not like to suggest which one!

What I call Canterbury Bells - white.

I will as I can see one of it's leaves, it's C. persicifolia (Peach-leaved Bell Flower).

...the white form 'Alba'.

Thank you all very much.......especially Fractal for knowing exactly.

I've had this plant growing at the outside bottom of my greenhouse for the past three years and I also was curious to know what it was, so an extended thanx to Jonathanhales, moongrower, bulbaholic and Fractal.
You guys/gals are sooo good!