Refusing collect and pay

Hi, my ex and I have been divorced for sometime now. We have 2 children together. Initially we had a personal agreememt were he was paying 350 a month but then he stopped paying becauae I was still living in our house with his name on the mortgage. Stopping the payments made me unable to pay the morrgage so now im in arrears. I wernt througb cms were they calculated he pay 380 a month which he paid. Ff a year to the cms annual review and becauae his wage increased he had to pay a little more, but he just kept at the 380, I rang cms and told them he hadent been paying the correct ammount and eventually I got a back pay from him. I have a partner who only comes over at weekend, he puts abit of money into my account monthly and also pays some of our sons childcare with help of vouchers. I work as a nursery nurse 3 days a week, during school hours as I just cant afford any childcare and im falling behind with all my bills because im just about scraping enough money together to pat my mortgage. CMS did another annual review this october and as my exs wage went up quite alot the monthly payment due to my children also went up over 100...cms advised that my exs weekly income was over 1000.
My ex has rwfused to pay the revised amount and wants to go back to our original plan of 350 whuch I just wouldnt survive on, certainly wouldnt keep a roof over my childrens heads. Hes asked the CMS to do a mandatory reconsideration as he says his wage is high because he does alot of overtime. I also recieved a letter from his solicitor stating im to take sole ownership of the house or sell in order to get his name off the mortgage, need to show evidence of this within 14 days or I will be sent to court with all costs being put on me. I am in no position to sell or buy my property...I just dont know what to do.