I would get some advice if i were you, my sister is in the same boat, her husband left her with two boys to live with another women!!, his name is on the mortgage but he does not pay, she can't get a mortgage on paper but with tax credits etc, she is managing to pay all the bills, the solicitors have said that his name will be on the deeds still as they are still married, but he will be entitled to a much smaller share should the house be sold in the future, she would love him to be taken off, could your mother take on the other half of the mortgage??, I would go and speak to the Ctizens Advice.
Good Luck!!