Help me help my mum

Mmmm. Did she ever tell the CSA of the change in her circumstances i.e. lost her job? If she didn't then the assessment has to stand and she will have to pay. If she is no longer working and your sister is still at school, your mum needs to give CSA the evidence of her circumstances and ask for a new assessment to be done. If they calculate a nil assessment now, she may not have to pay the arrears until a later date as they are usually very unlikely to enforce payment of arrears when an assessment is nil as they are deemed unable to afford it. She needs to ring them ASAP and 1. ask for new assessment 2. Make a temporary arrangement to pay a small amount per week until they have done the re-assessment and then ask for another agreement to be reached, or suspend collection of arrears until her circumstances change or if she feels she can still pay the small amount, continue to pay in order to just get rid of it. SHE NEEDS TO DO THIS URGENTLY.