Emergency replacement of a cylinder on a like-for-like basis would mean that the installation is no more unsatisfactory than before, so it would not be classed as a material alteration and therefore not subject to building regulations as long as the work does not adversely affect the energy and safety controls.
However, where reasonably practicable, it would be good practice to provide the system with overheat protection as described in the Approved Document, including a non-resetting energy cut-out to any immersion heater. It would also be good practice to check that the support of any cold water cistern supplying the system complies with the guidance given in paragraph 3.15 of the Approved Document.