Apologies for late update, long day at work.
Following advice on here, I sent DD with a note to Mrs X with half lines completed, explaining that she wasn't able to finish them due to lack of time, other homework, etc.
I called the school during my first break, and asked to speak to the year head, who wasn't available so I left a message explaining the situation. I was assured that I would get a call back later in the day.
I did get a call, but I missed it due to not being able to have my phone with me. The message left basically said that the matter was being looked into, and it would maybe be easier to either go into school to discuss, or email my concerns. I don't know which would be better?
Having looked at the school website, there is no specific guidance regarding uniform rules outside school hours. The rules for during school hours are very prescriptive, although from my observations there are students who don't observe all of them, as to whether they get sanctions I don't know.
DD saw Mrs X, gave her the lines and note, which were accepted, and a comment along the lines of "I hope that this is the end of the matter" was made.
I don't know to what to make of it all. Teacher on a power trip/off day? Or is is something more serious? DD seems to have had a good day at school, went round to another friend's after school and I picked her up on the way home from work, went via a pizza takeaway and she went in to get the order, still in school uniform and she seems back to her normal chatty self.
Would an email or in-person meeting be better?