After loads of two-ing and fro-ing, and letters, texts and telephone calls from the Chief Exec of the Mediation Company, I finally had a mediation appointment with my exH today.
I had decided that I wanted to address two issues - one was just to tell him that it was my perception that DD was unhappy with some aspects of their relationship and that I felt that they were growing apart. I've tried to talk to him about it before, but he gets defensive and deflects it back on me.
He did the same thing today - which I knew he would - but at least I've tried.
The other was to address the fact that despite adhering rigidly to the schedule, DD has missed out on spending time with him because he refuses to be flexible and he places her in the care of other people. I proposed that we maximise the time DD gets to spend with each parent by being flexible. He agreed, as long as it was still equal 
He is adamant that he will not agree to her spending more than 50% of her time with me, even if that means she misses out on time with him. He would rather she spent a week of the holidays in the care of distantly living extended family and didn't see him at all that week, than spent the days with me while he is at work and evenings/nights after work with him. 
I don't feel as though we got anywhere, and have no expectation that things will change.
But at least I tried.