"Earth" <==> "Ground" - different terminologies.
"MET" = Main Earthing Terminal
From Main Earthing Terminal (see pic. MET ), beside the Consumer Unit (distribution board) downstairs, through ceiling to upstairs. So for duration of renovation have an easily-used "wander-lead" to check Earthing upstairs.
Only the basin plumbing was Earth-bonded.
Impedance so low the meter cannot measure it - under 0.1 Ohm including my 10mm^2 "wander-lead" = good.
Did find these were cross-connected in an equipotential bond of very low impedance. The very visible "Earth bonding cables" (sic.) were equipotential bonding - but had no onward connection to Earth.
Basin Earth-bonded but no others - two Earth-bonding cables back from bathroom to MET, and the basin had been correctly connected to one of them.
Not mentioned is
Earth cabling is 10mm^2
In detail; would-have-been nearest connector removed. Labels added by myself as traced the cables with meter.
There are two 10mm^2 Earthing cables going back to the MET, routed under the upstairs hallway / corridor. Giving sum of 20mm^2 of bonding conductor back from the bathroom to the MET.
Other cables in from
Not in reality Earth-bonded...
"The other" Earth-bonding cable onward to MET has been connected one-to-one to the "mystery" cable.
While the other one-to-one connections of all other Earth-bonding cables form an isolated local equipotential network.
The bonding cables from the bath and cistern are recognised.
Local version of the MET at this point to connect all earth bonding cables in the bathroom...
A conductive metal block which conductively cross-connects all
Earth-bonding cables together "as one".
This is an Earthing circuit - everything Earthing is being "pulled" to the same potential and ultimately to Earth potential - there is no reason here for separate circuits.
This does the safe right thing with the "mystery" Earth-bonding type cable; whether is supposed to
An over-the-counter MET block, slightly trimmed at sides losing mounting screw holes, so fits into box
Seemingly works perfectly - all impedances from accessible "extraneous conductive parts" to MET including the wander-lead resistance back are something like 0.2 Ohm down to showing 0 Ohm (too low resistance / impedance to measure). For : towel-rail; basin; bath plumbing; cistern plumbing.
Defered until here because could not resolve whether shower needed to be protective-bonded to MET potential.
Water into home comes through a polymer ("plastic" / MDPE /
"Alkathene") pipe - cannot conduct from mishaps in outside world.
Then more "MDPE" pipe within home - electrically isolating the shower according to resistance-meter readings.
Which suggests protective Earth bonding of the shower might not be necessary.
There are no circuit terminals, hence no circuit protective conductor terminal, within reach of the shower...
In "18th Edition" - the current basis of electrical installations Regulations - these are the contradictory impressions I formed
Cable pulled with leading rope through drilled hole in bathroom floor under bath, into same between-joists void which has the shower. Being drawn towards shower
Access to that between-joists space from this access hatch in the kitchen, giving access to shower wastewater debris trap.
Earthing/bonding clamp in-place on copper pipe just before shower.
Run of connected protective bonding conductor back towards the bath.
View of hatch showing the protective bonding conductor (10mm^2 copper Earthing cable) clipped to joist.
Two thicknesses of plasterboard forming a barrier behind the plastic hatch.
Local Earthing terminal under bath, connecting the electrically bonding cables.
Meter gives Impedance from shower to MET and back through the "wander-lead" of less than 0.2Ohms.
(R. Smith, 11Oct2023, 12Oct2023 (edits, implemented), 19Oct2023 (shower))