Kevin Brady wrote:

I thought this confirmed that the earth is routed to earth via the mains
sheathing, [...]

Well that's what I thought too. You said

1 to the incoming mains cable, with a metal tag saying DO NOT REMOVE.

which seems fairly clear evidence for a TN-S earth to the service cable.
You just need to move the original water bonding connection onto your
copper pipework as near as practicable to the stop-cock and you're done.

Oh, and also

main fuseboard (not fuses but rcds - push button type)

them's MCBs not RCDs.

--
Andy