Flower friday circle

A busy day beginning with a spin class then after lunch some more planting on the allotment.  I took a few photos of my neighbours allotment which is covered in poppies but with the sun shinning full on them and the breeze they weren't up to much.  So this is one of the few flowers in our rear garden, it forms part of the hedge and I'm not sure what it is.  The outer white sepals form a circle around the white circular flowers in the center.