To clarify your issue regarding responsibility.(posted 7534 days ago)The LAW says that if you sign a Joint and Several Liability agreement, which if you were mentioned on the mortgage you have (The mortgage deed and or mortgage offer) then the lender can pursue any party in the agreement for even the full sum outstanding if they so wish.
Mortgage lenders do not often remove names from the deed as it is more risk, i.e. in your case they would only have the ex and his father as a method of recompense.