Court order to pay mortgage - ex pays no child maintenance

Hello All,
Been divorced 10 years and there's a court order on the property.
Ex has to pay mortgage until son completes his full time or undergraduate education.
Been told I should apply for child maintenance. This came up a few years ago too. However, ex claims he spoke to CMS and was told the payments would be offset against any monies owed for child maintenance, and as the mortgage was more, would pay nothing.
Is this true? How do they calculate this?
I'm currently working two jobs over 6 days to pay all the bills where only my son and I live.
Thanks for any advice.

It is correct they would take the mortgage payments into account. Whether that would reduce the maintenance to zero would depend on the figures in your case.

By paying the mortgage on the house your ex is supporting his child. Do the terms of the court order allow you to move? If the current house is bigger than you need perhaps it may be worth looking into downsizing. What is happening to the house when your son finishes full time education?

Thank you for replying and my apologies for the tardy response.

The terms allow me to move, and will remain in force until the trigger point of when my son completes his full-time education. Then is has to be sold and the proceeds split.

This brings other factors into play which haven't been addressed: what if it doesn't sell? Am I meant to pay the mortgage then? The order does not state anything.

Your ex will almost certainly stop paying as soon as possible and want the house sold as quickly as possible after the trigger event.

Realistically with nothing mentioned in the court order you will end up paying the mortgage during this period. The argument would be that you have sole use of the house and aren’t paying him rent for his share of it.