How did this happen, your name not on the deeds.
And she's some tulip not willing to sell the house, which is in positive equity but not pay her mortgage.
Have you assets or a good job? Your best bet is to contact bank and get them to force her to sell. What on earth is she playing at, ruining your credit history.
Can you tell us house value, mortgage repayment amount, arrears and interest rate.
You do not want it to run into NE by her not paying the mortgage, and also risk the bank piling on interest rate penalites and legal costs.