www.moneysavingexpert.com/loans/debt-help-plan
Firstly, have you had any financial / debt advice? Start with the CAB or a free, government advice line, there are a number on the web which you don't have to pay for, please don't use a paid service, you really don't need to. I've put one site above, there are a number of suggestions on here & various links & names of other organisations & debt advice charities etc on there.
It doesn't seem a great idea to refuse to pay the mortgage until they cooperate - they don't actually care & you're just increasing the arrears more quickly. Better to keep in touch with them & show you're trying to sort things out.
If they are refusing to comply with any terms of the mortgage, or conditions laid down by the court, complain to the court or put a formal complaint in to the mortgage company. Use the words "Formal Complaint". You can do this verbally or in writing. Be clear about exactly what you are complaining about, they have to take it seriously, investigate & do their best to resolve your concerns.
I presume you've chased up maintenance from your XP - does he know what is likely to happen if he continues not paying, that his DD will lose her home that's been adapted for her?
Suggest you contact the council/housing office also, depending how far down the line you are re: possible repossession, to look into alternative accommodation.