I went through so many permutations and undid and redid and shouted and swore, but I cracked it. And I love it (and so does my mum, fortunately).
1) I insisted on using stitches that were new to me, and were hard to reduce for the shoulders
2) The colour scheme isn't exactly safe
In the end, though, I think I got the shape of the shoulders right and I'm happy with the proportions of the three colours. My mum has bright blue eyes (she's not the one modelling it for these pictures, by the way!) and so the blue colour collar looks fabulous on her.