The Medway Handyman wrote:

What would be the purpose of an earth cable fixed to a brick wall?

Your guess is as good as mine, unless it's connected to any structural
metalwork buried in the wall (unlikely in this case I'd have thought).
Whatever its purpose, the connection isn't compliant with wiring regs
since it's not labelled with the essential words "safety electrical
connection - do not remove".

As an aside, I often see the copper feeds to sinks, basins etc earth bonded,
with flexibles connecting to the taps. Would this earth the taps?

1) Terminology: don't talk about "earth bonding" it just confuses the
distinction between earthing and (equipotential) bonding:

- earthing is about connecting the metalwork of electrical equipment to
earth. It reduces shock risk by reducing the /duration/ of dangerous
touch voltage resulting from a fault, by blowing a fuse or tripping an
MCB or RCD within a prescribed time;

- bonding is a more passive measure to reduce the /magnitude/ of touch
voltage arising between both electrical (exposed-) and non-electrical
(extraneous-) conductive-parts, or any combination thereof.

2) Taps don't need to be earthed or bonded, only the metal pipework
leading to them if it's able to import a voltage from outside the
location. In a bath/shower room you need to bond incoming metal
pipework and all electrical circuit earths. Short copper tails in a
primarily-plastic plumbing installation don't need to be bonded, and nor
do basins and sinks that aren't in a bath/shower room.

When boxing-in under baths, basins etc. remember that the bonding
connections are supposed to remain accessible for electrical inspection
and testing (unless soldered).

The 17th ed. regs (BS 7671:2008) do remove the the requirement for
supplementary bonding in bathrooms, _but_only_ if all circuits feeding
loads in the bathroom, including lighting, have 30 mA protection _and_
the installation's main bonding is present and correct. It is not a
licence to remove or ignore the supplementary bonding in existing
installations.

--
Andy