How much maintenance should an absent father pay for two children
under 18 under the CSA criteria/rules. I understand it is calculated
according to his income. He earns £108,000 gross p.a. (£73,000
take-home p.a.)The mother looks after them entirely alone, the
children were born within the marriage (ended in divorce.) Please keep
the figure for the two sons together and then just one boy when the
elder reaches adulthood next year.

Hi pearlrivers,

There is a calculator at the Child Support Agency website for
estimating child support payments. They use the amount of weekly
income of the nonresident parent, so you will have to use the income
calculator first before you use the Child Maintenance Calculator. Full
details are contained in my first link.

Child Support Agency
New Rules: Information for our customers who have a child maintenance
calculation made under the new legislation, after 3rd March 2003
"This section will show you how we calculate your child maintenance
payments. It also provides a calculator to help you work out how much
child maintenance is likely to be due. Please note that this web site
can only give you a guide to how much you will pay or can expect to
receive and should be used as a rough guide only."
http://www.csa.gov.uk/new/calculate/

Here's an example, using the figures you gave:

Income Calculator
Please Enter Your Gross Income:  108,000
Please Enter Your Income Tax: 35000
Your weekly net income is: £ 1403.85
https://secureonline.dwp.gov.uk/csa/v1/new/netcalc.asp?termbutton=annual&Gross=108%2C000&ITax=35%2C000&NICon=0&PensCon=0&submit=Calculate

Child Maintenance Calculator
You told us:
Number of children maintenance will be paid for: 2
Number of other children living in the non-resident parent's household: 0
Number of nights, on average, the non-resident parent has the
child(ren): Less than 52
The non-resident parent's net weekly income: £ 1403.
The weekly amount of maintenance to be paid is approximately: £ 281 

You told us:
Number of children maintenance will be paid for: 1
Number of other children living in the non-resident parent's household: 0
Number of nights, on average, the non-resident parent has the
child(ren): Less than 52
The non-resident parent's net weekly income: £ 1403.
The weekly amount of maintenance to be paid is approximately: £ 210 

Old Rules: Information for our customers who have a child maintenance
assessment made under the old legislation, before 3rd March 2003
http://www.csa.gov.uk/old/

I was glad to find this for you. If you have any questions, please
post a clarification request and wait for me to respond before
closing/rating my answer.

Thank you,
hummer

Google Search Terms Used: csa child support

Thank you so much, Hummer. I am very grateful - too broke to tip alas
because ex is paying me almost nothing!!! Armed with this, I might be
able to change it. But thank you for the help, Pearl