Argh this is great (though diasppointing) intel on the M and S one @evilharpy and @Cantonet and confirms my theory that what I want is incredibly hard to find!
Needs to be boxy, which doesn't work if the material is too thick/ heavy OR too finely knit as either of those means it won't do the boxy shape thing properly
Needs to sit on the hip rather than lower as then tucking becomes complicated and after three children I certainly don't need additional padding in that area
...but not be actually cropped - as I find this is problematic when you raise your arms for any reason - just rides up and shows your belly. Also tends to lose shape easily when washed
It was the attached image on the M and S website which made me think it was the right fit but maybe the images are deceiving!
Also thanks @BeckyAMumsnet for correcting my CF title before I got any more Bob-Geldof based abuse