CSA maintenance for the children is always a requirement under UK family law until they are 19 or out of FT education whichever is sooner. It does not matter what agreement is in place surrounding the FMH he must still pay CSA.
It''s swings and roundabouts. He has a CM liability, but if you are enjoying sole use of the former matrimonial home it it could be said you should pay your share of the mortgage and his share for occupying his share of the home. If that leaves you struggling financially and he has the ability to help out once he has met his own living costs he may need to help you out with the mortgage.
Be aware that if you go to the CSA for CM, he may stop paying his half of the mortgage. As Fiona says if you are the one occupying the family home, then he could argue you pay all the mortgage as you are enjoying the sole benefit.
Do the calculations and see if keeping it this way he is contributing the correct amount.