This afternoon I set to work with the scissors and sewing machine and took a couple of inches off the sides, removed the single cap sleeve, stitched a belt and set to work with some pins on the yoke.
Already 100 times better. The problem with the boxy yoke and sleeves is probably down to the fact that the knit I chose is quite thick, so it's never going to drape nicely as a double thickness. Still undecided about whether to try and draft a sleeve pattern myself (a bit scary!) or maybe trim the yoke to how it's pinned in the second pic and bind it. Getting there slowly. I'm hopeful of having a wearable dress at the end of it.