Not sure what it is. However Fat hen can be ruled out as the leaves are not the right shape. Chenopodium album...common name fat hen has leaves that are more triangular in shape...see below
The leaves look like Silene latifolia ... White campion, a gorgeous native wildflower and great for pollinators, particularly nightflying moths. Have you sown a wildflower mix for pollinators there in the past couple of years?
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However Fat hen can be ruled out as the leaves are not the right shape.
Chenopodium album...common name fat hen has leaves that are more triangular in shape...see below
https://www.google.com/search?q=Chenopodium+album&rlz=1C1CHBF_enGB785GB785&sxsrf=ALeKk02h_WxFC3oywDEtLehjiPicnUv4g:1617697411610&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjYn8q8mOnvAhWugf0HHbJJDYgQ_AUoAXoECAEQAw&biw=1280&bih=578
https://www.plantlife.org.uk/uk/discover-wild-plants-nature/plant-fungi-species/white-campion
So much easier to id when there are flowers.