A friend emailed to advise that it's worth checking whether child tax credits for the tax years ending 02 and 03 were paid. I'd better mention that this applies to UK tax payers with families. There is a deadline 31/01/09 I think.
It works like this:
1) Find your P60s for 2001-2 and 2002-3 and note the Final Tax Code.
2) Ring 0845 302 1463 and ask whether the Children's Tax Credit was applied for those years. They'll ask for your NI number then the Final Tax Codes for those years and will tell you whether or not you've already had the credit.
3) If not, they'll ask you to write in enclosing the P60s, giving details of your children's names and dates of birth and asking for an assessment due to Children's Tax Credit not having been applied.