The basic one piece collar I'd drafted was nagging at me. I could see it in my head, and it looked wrong to me (bear in mind that I've only seen this in my head - I have yet to sew any of this coat together!) So despite the fact that hardly anyone would notice or care, I redrafted a proper two piece stand and fall collar. I feel so much better now! Here's the process, following instructions found in Winifred Aldrich's Metric Pattern Cutting for Menswear. My copy (bought on a bargain table - SCORE!) is the 2nd edition.
Firstly I traced my original collar since the two piece one I wanted is based on the same style of collar. The darker line in the picture below is the original collar, and the lighter line (pretty hard to see, sorry) shows the new collar size, and the stand. Since most drafting is based on measurements you plug into a formula it was easy to use these instructions for my new collar. I did have to scale back where they gave a measurement of 4.5cm for width of the stand - obviously too big for a size 2!
Next step is to cut the pieces apart and slash them vertically to open up the outer edge of the collar and close up the top edge of the stand. Again I scaled back the overlaps and spreads to match the scale of much smaller pattern pieces.
This was not something I was prepared to construct without trying first, so I cut a quick mock up of the collar pieces from some scrap fabric I had to hand, sewed them together, and pinned it to the pinned together fronts and back of the coat.