Do I have to pay maintenance and the mortgage for the family house they live in

If an application is made to CMS or already exists with CMS/CSA you are legally required to pay.

If you're named on the mortgage then you have entered a legally binding contract with the mortgage provider to pay the mortgage. If the mortgage is just in your name, you are responsible for paying all of it. If it's in joint names, you are both jointly and severely liable.

Of course, you could stop paying one or both, but there would be consequences to both decisions. You would then have to decide if you are happier with the consequences than paying both.

You can't just walk away from a mortgage because you've split up. You will need to either work with your ex to sell the house or for her to buy you out or for you to pay off your share of any negative equity and her to take on sole ownership, so you are legally free and clear to walk away from that commitment. If you can't or won't, your only option is to take the matter through the courts.

If you do pay both, you can ask CMS/CSA to take account of the mortgage payment as a variation to your child support calculation.