Statutory Instruments (SIs) are regulations made by a Minister under the special powers conferred by an Act of Parliament. They are given a running number throughout the year, for example SI 97/103.
You can search for UK Statutory Instruments on legislation.gov.uk or using Halsbury's SI Citator which is one of the sources available in Lexis+.
We also have the printed Halsbury’s Statutory Instruments at Main Library Periodicals K 35.5 on Floor 3.