I am in a joint mortgage with my now ex partner, our house is on the market but I have been living there since he left in May 07, I have now just moved out to start a new life and just wait for the house to sell. My ex and I have been paying half each on the mortgage, however he has just told me that he is going to move back into the property with his new girlfriend, should I still have to pay half, I don't think it's fair that I should be paying for her to live there. We are also in an IVA so either of us buying each other out isn't an option. Should I offer to pay a third or am I legally bound to pay a full half?

You shouldn't have to pay anything. The house is community property and as such, upon the sale, you would be getting half of the proceeds.