Well, she doesn't have to go to mediation - its not compulsory, although its a pity if she doesn't. I think if she has refused you have no choice to to begin the court process for AR in the hope that by starting, and showing you mean business, she'll realise its in her benefit to co-operate and negotiate a fair settlement.
Our mediator explained that it is cheaper to thrash out your differences at mediation than to do it in court.
We knew what we wanted, and to be honest I was making it as easy as possible, so we only needed one session, which cost 170 pounds. I know that is unusual. Court time and all the solicitors and lawyers are VERY expensive, but when it comes down to it, mediators help you to decide what you want. If you can't agree, then a court will decide for you, and you may not like what they decide.
To the original poster... maybe she thinks she will get her own way if it goes to court.