Competent Person schemes (CPSs) were introduced by the UK Government to allow individuals and enterprises to self-certify that their work complies with the building regulations as an alternative to making a building regulations application.
A Competent Person must be registered with a scheme that has been approved by DLUHC (Department for Levelling Up, Housing and Communities) Schemes authorised by the DLUHC are listed on its website.
An installer registered with a Competent Person scheme is qualified to carry out specific types of work in accordance with building regulations and should both notify the local authority of the work and issue you a certificate of compliance with building regulations either directly or through their scheme operator.
If you don't receive a certificate within 30 days of completion contact the Competent Person Scheme operator that your installer is registered with and they may be able to help you resolve this. A Competent Person can only self-certify within the scope of their registration scheme.
N.B. the local authority can't issue a completion certificate until they're in receipt of the Competent Person's certificate.