Wondertape eyy. I'll hunt it out and give it a go. I like the idea of it washing away so it can't be seen!
Lots of really good ideas here thank you. I'm not sure I'll buy such a lightweight knit again for a while, it was quite stressfull to sew with because I was so worried about stretching it and it was flopping all over the place plus the woven I used (which isn't that heavy) seemed so awfully heavy once I'd sewn it - it's definitely affected the neckline.
@Bodgeitandscarper the armscye was actually really well drafted - it's not snug at all. I think it looks tighter as the woven is soooo much thicker than the knit and my binding is always a little on the "I've folded this too many times I think" side