Everyone has heard of it - but who knows what the Child Support Agency does or how it operates? Jonathan Stayton went behind the scenes at the CSA's South East base in St Leonards to find out.

About 18,500 parents in Sussex owe a total of £71.7 million in maintenance for their children - that's an average debt of £3,876.

Nationwide, one parent in three who should be paying maintenance is failing to do so.

The CSA is using everything from phone calls to fines, bailiffs, registering debts against properties, court orders and subsequent prison sentences to urge debtors to pay up.

Agency staff know they have a poor reputation but say they are working hard to help children who are suffering because their parents' relationship has failed.

The CSA assesses, collects and pays child maintenance, ensuring children are supported financially by both parents even if they do not live together. In the past 12 months, £1 billion was collected in Britain. In the 14 previous years, the CSA collected £6.5 billion in maintenance.

In the year up to March, there were nearly 750,000 children benefiting from maintenance payments, almost 200,000 more than the same period three years earlier.

Principal legal enforcement officer Stuart Richards said the amount of money owed in Sussex was "a shocking figure".

He said: "Much still needs to be done to collect money from the non-resident parents who refuse to pay and our enforcement powers have been strengthened to do this.

"We give the parents every option to pay but where they don't, we pursue that and do all we can to help them pay before registering a debt.

"We don't want to have to take this action. We want it sorted as soon as possible.

"Our role is about making the communication and making them aware."

Last month the CSA secured its largest single child maintenance payment - £57,600. It followed 12 years of trying, including using charging orders on the parent's £500,000 property.

Last month, a mother in Shoreham received a cheque for £18,000. One week after a visit by senior legal enforcement manager Julie Collins, the CSA received the cheque for the full amount the father of three owed. His children are now all adults.

Ms Collins said: "The mother was on the brink. She had an eviction order on her home. The fact the so-called children are now adults tells you how long this man had not paid for.

"She had spent all her money on feeding and educating the children.

"In another case the child was 22 and about to have her own child. She had not had the support she should have had and went short for most of her childhood.

"You can go to a home and find people who have pawned all their jewellery, who have wallpaper hanging off, broken washing machines, broken cars - all because they can't cope on their income.

"It's a vicious circle. It can be really sad and you get some real sob stories."

Excuses for non-payment range from "I'm not the father" to "I can't afford to pay" and "I didn't even want the child". The CSA said 90% of those who owe maintenance are men.

Typically, parents can expect to pay 15% of their net income in maintenance or 25% if they have more than three children.

James Plaskitt, the minister responsible for the agency, said: "The CSA is delivering real improvements that are benefiting children and families across the South East. We are collecting more money for more children and continue to target those parents who fail to face up to their responsibility."

"Parents failing in their responsibility to their children should be under no doubt - the agency has strong powers to recover outstanding debt, ranging from the suspension of driving licences, freezing assets and the sale of goods and property as well as prison - and we use them."

Senior compliance manager Barry Porter said: "We target all those who fail to take responsibility for their children. It's important to remember this isn't about securing money for the CSA but about recovering money for those children who are owed it.

"Some parents think they are paying for luxuries but that's not the case. They are paying for their own children to have a reasonable life and a decent upbringing. To non-compliant non-resident parents, the message is clear - act now before we do. We will help you but if you don't pay we will pursue you relentlessly."

The CSA South East bureau takes about 120 people to court every month, soon to increase to 180.

Last Tuesday, 60 were due to appear at Brighton Magistrates' Court. Five cases were withdrawn when full payments were made. Only nine of the other 55 people turned up and eight went home before their hearing.