Good old Sainsbury's! I bought it in petite and it fits perfectly.
tuclothing.sainsburys.co.uk/product/tuc142189401
And it's £22, 100% cotton and manages to disguise my menopausal tummy.
Also, it works to size down, I'm a generous 10 and bought it in an 8.
Also, I promise I don't work for Sainsbury's 😂