Hi Gums.
Thank you for your post. My name is William I am the Child Maintenance Options consultant.
The Child Support Agency (CSA) works out child maintenance payments based on your income alone. This includes earnings, money from an occupational or personal pension and tax credits. They use the amount of income left after deductions such as Income Tax, National Insurance and any money paid into a personal pension scheme.
They also take in to account any Child/Working Tax Credits that you, or your partner, may receive. This is because it is classed as a household income.
The only time Working Tax Credits would not be taken into account is if your partner is the main earner in your household. However, Child Tax Credits are always taken into account.
Other factors are taken into account (including the number of children that need child maintenance, and if you live with any other children) but this does not include your new partner's income.
There are many other financial factors that may be taken into account by the CSA, so you may wish to speak with them directly about your change of circumstances.
Their contact details can be found on any letter they have sent you, or this website http://www.gov.uk/child-support-agency provides their contact details.
I can see actd has suggested you set up a family-based arrangement, this type of agreement is not legally binding but it gives you the flexibility to agree between you what counts as child maintenance, and to change your agreement as your children get older or your circumstances change.
For more information about family-based arrangements and access to useful tools and forms online you can visit http://www.cmoptions.org, or if you'd prefer a confidential chat you could call the Child Maintenance Options team on 0800 988 0988 (free from a landline).
We also have a sorting out separation web-app that you may find useful, it offers help and support to separating families. The link is: http://www.dad.info/divorce-and-separation/sorting-out-separation