I have made progress with my coat - er - my muslin.
Here again is the Vintage Vogue Christian Dior pattern I'm using

After I graded the pattern, which if you recall is described as "high - fitted", I made up a muslin out of hideous curtain fabric - it's slightly shiny and stripey which shows off the bias cut. Despite the flaws of the fabric, the coat looks pretty good. The only problem is that it is so fitted (quite nicely I think) through the shoulders and arms that there would be no way to actually wear it over anything.
Round two saw me add to the width in these critical areas. I extended the shoulder point and side seam at the lower armscye out 1cm each. I also lowered the armscye by 1cm. This gives me a total of 4cm more width just below the arms, blended into the original side seams around waist level.
I also made the sleeves wider to match the new armscye.
The only fitting change I made was to curve the CB seam in more at the waist/hip level as it was quite baggy.
Finally, I decided the coat needs to be 4cm longer to cover most of the skirts/dresses I would wear it with. I kept the original hem circumference.
Here are the pictures of my first muslin (tight jacket fit)


And here is the shorter & bigger muslin, worn with a jacket underneath.


I had better get going now. I have to finish the rest of the pattern pieces and cut this out. I have decided to use a mostly windproof underlining and kasha lining. I have hair canvas on hand, and I will decide about whether to use the lambswool I bought once I have the shell constructed and can get a sense of the heft of the coat.
Kay