So (prompted by advice from S&B folks) I bought (what I thought was) a lovely Violeta by Mango blue denim shirt dress from eBay. Fits beautifully, and a bargainous fifteen quid.
Alas, expectation vs reality is a wide gap! I imagined effortless 70s chic but in all honesty I look like an extra from Prisoner Cell Block H! I've already ditched the black skinny rib poloneck which was giving off definite Wentworth vibes, and added a slim back belt to give a bit of waist definition. Anything else I can try? Currently wearing it with black opaques and knee-length chunky biker boots - may risk a biker jacket and big bright scarf later.