Here we go - I'm much much happier with the fit of this one. I raised the neckline 1.5 inches, lowered the point of the bust darts by about 1.25 inches, adjusted the bust darts so that instead of straight legs they have bent legs (like in this tutorial by Gertie at Gertie's New Blog for Better Sewing), let out 0.5 cm from either side of the bust darts... and I think that's about it! The darts weren't done in the highly mathematical way as described in the tutorial - I tried doing it like that and still couldn't get it quite right, so I just pinched off the fabric into a good shape for my body and it turned into that shape so I went with it!
The end of the darts looks not quite right in this picture but I've since pressed and re-pressed and re-re-pressed them, and it's finally sitting properly.
In regard to my gapey back problem, I took about 1.5cm (I think?) off either side of the centre back seam, tapering down to the original seam allowance just above where the waist darts end, and I think that's just about fixed the problem without completely restricting my movement.
In other exciting news, I have acquired the fabric - it's a satin charmeuse, from Spotlight, and it's lovely! Plus, it was on sale for about $6.50 a metre instead of $12.99, which is brilliant considering how much I need - 4m of black and 4m of teal/peacock. Hopefully I've done all my calculations correctly and I don't need to go back and get more!
As before, any input on the fit of the bodice would be much appreciated, before I cut into my main fabric, hopefully sometime this week.