Take-up rate estimates are not available at constituency level.
However, estimates of child and working tax credit take-up rates for 2003-04 and 2004-05, which are the latest available estimates, are published on the HMRC website at
http://www.hmrc.gov.uk/stats/personal-tax-credits/cwtc-take-up.htm
Take-up rates for working families with children at country and regional level are shown in table 9.