"Lobster" wrote
| ... I've found two old green 6mm(?) earth lead which emerge from the
| plaster adjacent to the (ancient) fusebox and connect to a terminal
| block,

Those might disappear off to water and gas pipes somewhere.

| from which I'd expect to find at least one wire heading for
| the fusebox, and another heading for the house earth, whatever that
| is. But as far as I can see(*), nothing; so presumably there's no
| earthing in the house at all. All I can find is a small, uninsulated,
| twisted wire dangling down from the terminal block, with an eye
| in one end, which once upon a time will have been connected somewhere
| I suppose.

to the fusebox, especially if it is (or an even older one was) metalclad.

It is also possible that there's an earth (ie bonding) from a socket in the
kitchen to the cold water main, and that, brought back through the circuit
protective conductor to the fusebox, was considered sufficient to earth the
whole installation.

Owain