One of the problems with the Government inventing lots of complicated tax credits and benefits is that people very often don't get what they are entitled to. One example is something called the "children's tax credit" (not to be confused with the 'child tax credit' which is what you can get now). This ran for just two years 2001/02 and 2002/03 and the deadline for claiming for 2002/03 is the end of January 2009. The 'children's tax credit' was introduced in 2001 to make sure that people with children did not lose out when the married couple's tax allowance was abolished in that year. It is too late to claim for 2001/02 but you can still claim for 2002/03.
It was worth about £520 (or double if you had a child born during the year) and was payable once per family. You could qualify if you paid tax during 2002/03 and were responsible for a child under 16. People who paid higher rate tax got less help, and those on the highest incomes got nothing.
If you think you didn't get the money at the time and might be entitled, you need to act quickly. You can download the claim form from here and it needs to go back to your local tax office by 31st January.