The diagrams for supplementary bonding in bathrooms etc given in OSG
fig 4d & 4e & in the "IEE Elec Guide to the Bldg Regs" figs 5.3a & b
are reasonably clear.

What is not clear is how much of the bonding conductor may be run out
of sight.

For instance, if copper pipes enter the bathroom at separate points
and connections between them are out of sight behind a ceiling or
under a floor (and in the case of the CH fed radiator a long way off),
can the linking yellow/green insulated bonding conductor be also run
out of sight?

In other words, how visible does supplementary bonding have to be?