First of all, Tyedye, don't leave. If separation is inevitable (and I'm not clear from your posts whether it absolutely is), then it'll be much less upsetting for the children if it's him who goes, not you.
Re. the mortgage, etc, if he does move out and stops paying the mortgage, make a claim straight away for housing benefit from your local authority. It'll take forever for them to start paying anything, so in the meantime tell your lender what's happened and ask for a payment holiday.
Also make a claim either for tax credits if you're working more than 16 hrs pweek, or income support if you're not. The I.S. people will get the CSA to make him pay child maintenance.
The message from me is, don't let practical/financial stuff stop you from ending the relationship if that's what you really want. There are systems in place to help you and your children in these circumstances.