In reply to jockk:
I think the mountain hardwear xl is American 16 which is UK 18. Worth trying on.
I would also look at Berghaus, they go to 18.
I personally find berghaus and mountain hardwear best for me, I have curves and I find they fit well. I can't wear clothes from some outdoor companies because to get them to fit over chest/hips requires 2 sizes too big for the waist.