I'm replacing an old section of cable to a light switch. The position
of the existing cable does not comply with the 150mm vertical channel
above the light switch, as stated in my BS7671 on-site guide. It's not
an option to move the cable because of some period architectual
features, so I've been forced to buy a length of stainless-steel
trunking to protect the cable.

The trunking will eventually be buried in the plaster, but I'm not sure
how to earth this section. Can I just take a earth feed from the light
switch and bolt it to the stainless-steel? Or do I need to take a
separate line from the earth on the CU, in which case does this work
become notifiable?

Thanks in advance