As I said yesterday - I don't understand why the question of the mortgage payments wasn't dealt with from the outset, when you separated, albeit that it's taken a couple of years to get your ex to agree the terms. If there's a financial settlement already taken place, the mortgage payments should have been included with all the other household bills, IMO. I don't know what you can do about it now, if all the settlement for your split has been agreed. I know this isn't helpful to you, but I think it's too late.
If you were to take her to court now (if you can), is she still going to be living in the house, how would you get the payments from now onwards?