Help with difficult "house" situation?

My girlfriend and I want to get a house together, but, she can't get a mortgage because she already has one.

In 2004 she built her own house with her ex partner. In 2008 things had very much broken down in the relationship and we suspect he was having an affair, They agreed to carry on living together and to put the house on the market. During 2009 my girlfriend moved out after an incident of domestic violence.

At end of 2009 we got together and today, my girlfriend lives with me and we are very happy together, we live in a house that I lived in with my ex partner, so she isn't totally comfortable and that's one reason we want to move and get a house together. Over 2 years later her house is still on the market, there have been 3 separate offers which have all fallen through. Her ex partner has another girlfriend who is living with him. My girlfriend is continuing to pay half the mortgage for a house she isn't living in, whereas he is only having to pay half the mortgage for him and his girlfriend to live in a relatively new 4 bedroomed house which was valued at £250K.

My girlfriend has asked him to give her £50K and to take over half the remaining mortgage of £45K, and she will walk away but he says he can't afford this.

He has already won with every aspect - using and hurting my girlfriend whilst they were together, keeping most of the possessions because it was my girlfriend who had to move out due to the situation, living in a nice house and only having to pay limited mortgage. We want to salvage something for our future, ideally we need the house to sell so we and especially my girlfriend are able to move on with our lives.

Please can anyone help us by giving some good advice as to what can be done?

Is there a way we can make the house sell, or force him to buy my girlfriend out?

Is it possible for my girlfriend to charge any rent?

Have you any other suggestions?

Thank you for any really good advice, I am not looking for "go see a solicitior" advice, but anything else, or if we did go through solicitors,a short explanation of what they would be able to do?