[Solved] Ex moved children away whilst in Afghan

My wife and I split up about 6 years ago and she lived local to me with my two children aged 9 and 14.
I had to go away on tour in Afghanistan last December and whilst I was away she moved the children away without letting me know where they were going. I had no contact with my children whilst posted overseas as she refused to reply to my emails, phone calls and letters. I only found out that she had even moved away when my now wife told me the house that my ex wife was living in was vacant.
I managed to track down where she was through a PI and discovered that she had relocated some 200 miles away. When I came home on R&R i paid her a visit to try and find out what was going on. We talked, and i thought that we had come to an understanding about what we would do when i returned from duty in November. This was until my wife called me in Afghanistan to advise me that I had received a letter from my ex wifes solicitor. It claimed that my ex wife had advised that I was a threat to my own children. This is nonsense, I have never been violent towards my children or my ex wife, I have always taken care of them, had informal joint custody of them with them staying at my home for two or three nights a week, and have paid maintenance to my ex wife since we split up.
I am now home in the UK. I am being forced to go to mediation before I am able to see my children I have not seen or heard from them for three months. I am unable to get any information from the new school about how well my daughter is doing as i am not registered there as a parent, although I have been told that my son remains on a waiting list for a school in the area even though this is the year he should be studying for his GCSEs.
I am at my wits end. Can anyone please help?