17th February 2011: Final fix The boning idea didnt exactly work out. :S
I finished making the boning parts and tacked one against the long side seam, then tried the costume on. That was when I discovered I would not be able to bend at the waist, ha ha ha ha. Duhhhh. Now I know why corset boning stops at the waist!
So, next I cut the boning in half so it finished at the waist, tried it on...hmm it did hold the cossie stretched but was poking out at the waist now. This is why corsets are NOT made using thin stretch lycra! Boy, fast learning curve with this one!
Finally it dawned on me the pulling down problem started only after the swimsuit back was sewed onto to the elastic back strap. So I unpicked the back seam and tried the costume on. The swimsuit back sprang down leaving a gap, but it wasnt pulling on the whole costume any more. *Huzzah*
Final fix - so I cut a fill in piece of black lycra and sewed over the gap between swimsuit and elastic strap. ..problem solved. Phew. Back on course..
Pic - the grey area shows the gap between the back elastic and swimsuit. I tried to photo the lycra patch but since they are both black doesnt show up well, so heres a sketch.