Doing my research I came up with a few images.
This one shoes a detachable collar detached and yay I can see the back. And this one the collarless shirt.
Looks like if I add a button hole to back of the collarless shirt and corresponding button on the stand up collar then find some way of attaching the front. Maybe I can cheat and used a split pin?
I also managed to take a look at some collars at the Beamish Museum and ask a few questions on holiday. Should have taken come close ups really. Also stumbled across a blog post from someone making one (at last).
Looking at the top image and comparing it to a few book images. I think his style of collar is maybe Newmarket?
Next question is sleeves I have a shirt pattern in my stash with long sleeves and cuffs. eeep! Cuffs. One of my nemisis. Those will not be detachable though.