Bruce wrote:

I wrote:

So there's no code of practice that specifies exactly how
the back-box earth should be connected?

Correct, because as was stated in the first reply there is
normally no requirement to earth the back box.

Terminologically incorrect!

Back-boxes (flush or surface) are classified as 'exposed conductive parts'
and are therefore required to be earthed. IMO, earthing of the back-box is
mandatory.

You (and Adam in the first reply) obviously meant that there is often no
requirement to fit an earthing tail to achieve the earthing of the back-box,
not that the back-box itself does not need earthing. Where the cpc is a
cable or core connected into the socket outlet earth terminal, you can
(allegedly) 'rely' on the back-box fixed lug and the socket outlet earthing
strap and eyelet.

Personally, I would always fit an earthing tail, rather than rely on a
dubious combination of fixed lugs and eyelets.

--
Spike