Changes made by Welfare Reform and Pensions Act 1999 (Commencement No.10, and Transitional Provisions) 2001
S.I. 2001/933
Made to Welfare Reform and Pensions Act 1999 (Commencement No. 9, and Transitional and Savings Provisions) 2000
From legislation.gov.uk's record of changes, the official one: see it there.