I am sorry you had the letter.

"I don't know whether to just give in and let him have the D or fight it like I said I would originally."

What exactly are you fighting for Alison? No you don't have to answer just be sure for yourself.
We both know sooner or later your h will get what he wants, a D.
Please remember whatever happens down the road will be determined by what happens now. How you relate to each other as parents.
Also fighting is not the same as standing.To give in also does not mean giving up. You just abide by the law do what is asked of you and no more.
I am not saying give up just to think that this acrimony could go on for years and for what.
I would speak to you solicitor and let her do the letter writing with what ever you decide.
I am sorry this happened so close to your birthday. Never a good time I guess.