It sounds like you may have not worn a non-stretchy woven dress with sleeves before?
Exactly! I was quizzing my boyfriend last night about how much he can move in his shirt. We were out for dinner and I was getting him to do the YMCA!!!
Thanks so much everyone for the help. The issue is that I (we) like how it looks so I don't want to mess that up. I have, however, been persuaded to take it in a teeny bit at the waist (currently 3" of ease), which will mean I lose more movement so I think I want to try something.
I am going to try shaving a bit of the armscye. Does that mean I should increase the sleeve head a little bit too?
I'm also going to do some 'cut on gusset' experiements in the style of the flamenco sleeve.
I shall report back on how it works out!