The Joint Committee on Statutory Instruments (JCSI) is responsible for the technical and legal scrutiny of statutory instruments. Its members are MPs and peers. The same MPs are also members of the Select Committee on Statutory Instruments, which performs the same role for statutory instruments that are only presented to House of Commons.
If a statutory instrument is badly written or there are legal problems with it, the Committees draw Parliament’s attention to this.
They don’t look at the merits of the statutory instrument or the underlying policy. The House of Lords Secondary Legislation Scrutiny Committee, on the other hand, does look at the policy implications of statutory instruments. The House of Lords also has the Delegated Powers and Regulatory Reform Committee, which looks how delegated powers are used in bills and whether the right level of scrutiny is proposed.