Palmer Pletsch for McCall's 2818
Alma Aguilar version 2
And here is version 2, with a differently cut yoke and the same gaping problem.
Again, fits very nicely across the bust - hooray! I cut the yoke across the princess seams just above the bust apex and the neckline even deeper and wider and it does sit ok if allowed to sit very wide on my shoulders (much wider than I would like, strapless-bra required wide) and I haven't done the understitching on the neckline yet and it doesn't have sleeves, which do tend to pull things into shape, but I have run out of fabric until I can find another French Connection butterfly print top on ebay and ...
It also occurs to me that this is a semi-fitted pattern. I'm wondering if that doesn't lend itself to a deep neckline. Anyway, I'm going to put this one away for (maybe) later.
(Final lesson learned - maybe those late-night dressmaking sessions are not such a good idea.)