Chris Green wrote
| Marginal but, yes, I would think so unless there was access via (for
| example) removing a double socket to get to the junction box. If you
| just blanked off the old switch with a blanking plate then that would
| give access. However if you fill the hole and plaster/paint over it
| then it's inaccessible.

Also if the original switch is removed and plastered over the wiring may no
longer be running in the permitted zone up/down/across from a visible
accessory, so the wiring might have to be mechanically protected. This might
apply not only to the new wiring but also the existing.

I think I'd run triple-and-earth to the new switch and have two-way
switching. No doubt then that the cables to the switch are still live and
what they do.

Owain