Designer got back to me 'there are markings for the overlap'. There aren't. 'OK just overlap so it's the same size as the front'. Great. Very professional.
I've sewn it up as instructed and have realised that it's going to add even more issue because I want to fully line the dress, not just line it on the top. On this version the lining is just on the top and is loose at the waist. The back is stitched up to 2cm below the ending edge of the lining so the lining isn't concerned. But that's obviously going to be an issue for me.
I think I need to buy a pattern which is similar but fully lined and with a placket in the back so I can work out how to do this properly, but retain the style. Unless I can find a tutorial for this. I'm still totally lost as to what is the best solution here.
I'm thinking, with all things considered, the easiest option is going to be to go for DF's suggestion of just sewing it up like normal at the back and then having a keyhole. Except because I need the keyhole to be longer, I'm thinking instead of just one loop and button at the top, several down to the waist. So like 5 or 6 little buttons with loops. I can do them fabric covered and use thin ribbon to save myself making the loops. Thoughts? Also I was thinking of doing some sort of facing on the inside so that skin doesn't show through where the two edges meet.