Tbh mate the c&g2391 is built around gn3 and mine fell apart where I read it constantly. Actually a good one to help with it is a book called "practical guide to inspection, testing and certification of electrical installations" by KITCHER.
A good read with photo examples which helped but tbh just digest gn3 over and over
BS7671, the on-site-guide and guidance note 3, have sufficient information to pass 2391.Becoming conversant and familiar with these documents is essential to being able to interpret if and installation is satisfactory or unsatisfactory in its compliance with current regulations.