Child and working tax credits were introduced in April 2003.
I refer the hon. Member to the answer given to the right hon. Member for Birkenhead (Mr. Field) on 16 June 2006, Official Report, column 1421W.
Estimates for 2005-06 of the numbers of in-work families with tax credits awards, including information on overpayments and underpayments by constituency based on final family circumstances and incomes are due to be published in May, as set out in the annual scheduled updates section of the HMRC website:
http://www.hmrc.gov.uk/stats/updates/index.htm