A sort of update on this pattern!
I've traced the pattern pieces and am going to try various alterations on other fabric, but in the meantime I looked up Nina Lee who is the pattern designer. I decided to email to ask if the shoulder was supposed to be an extended one and said what I had done and gave my problem. Had a reply saying it isn't extended but is wide. She asked how I'd narrowed the shoulder so we'll see what happen.
If nothing else I feel I'm going to get to grips with this problem and intend to do a proper bodice sloper for future reference.