How can i make ex girlfriend pay her half of mortgage?

I split from ex leaving her in the house we held a joint martgage on. We are not in negative equity and both work. I have a job with a financial institution for which i need a good credit history. The house is currently on the housing market and she is living there with her child from a previous relationship. I have been renting elsewhere for nearly nine month and until recently, i continued to pay all the mortgage until i could no longer afford two sustain two properties. I now pay half of the mortgage payments and for the last two month, she has not paid. This is going to affect my credit score which i do not want to happen. I cant afford to pay and have been in contact with the mortgage provider who are aware that the house is on the market. They have advised to switch to a interest only using the house as a repayment vehicle. This is fine in theory, but she will not respond to my requests to pay, let alone sign anything to switch mortgages. How do i get her to pay without spending loads on a solicitor, and, should she continue and i get a bad credit rating due to this, will repossession proceeding commence, given that i have been paying 50%, the house is for sale AND i have made the mortage company aware of the situation!