It's a bit out of focus and can't tell the size but looks like a maiden pink (Dianthus deltoides.) If it's that, the flowers are usually about 1 to 2cm across and plants have short, thin leaves.
A trowel in the hand is worth a thousand lost under a bush.
This is the best I can do. I only have one photo. The flower was in the battlefields to butterflies garden at Hampton court flower show so I guess it's a wildflower. I did not the opportunity to ask anyone whilst I was there.
Posts
https://www.ebay.com/itm/Bright-Eyes-Flax-Linum-Grandiflorum-400-SEEDS-ANNUAL-FLOWER-/221613163130
thanks to all for replying.