Eu sei que você está no Reino Unido, mas eu responderei pelos EUA caso alguém tenha a mesma pergunta.
As caldeiras e fornos devem ser alimentados por um circuito separado e aterrado.
Qualquer sistema de tubulação de metal que possa ser energizado deve ser ligado, incluindo tubos de gás, mas o condutor de aterramento do equipamento com condutores de circuito alimentando um aparelho a gás "será permitido para servir como meio de ligação":
NEC 250.104
(B) Other Metal Piping.
Where installed in or attached to a building or structure, a metal piping system(s), including gas piping, that is likely to become energized shall be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor where of sufficient size, or the one or more grounding electrodes used. The bonding jumper(s) shall be sized in accordance with 250.122, using the rating of the circuit that is likely to energize the piping system(s). The equipment grounding conductor for the circuit that is likely to energize the piping shall be permitted to serve as the bonding means. The points of attachment of the bonding jumper(s) shall be accessible.