To bond or not to bond..?

[JohnW2]

With TN-S, the L&N obviously have to get to the upper-floor flats, anyway, so to also take the earth is not much of an issue, is it?

Three wires cost 50% more than two wires

They do, but that's back to your point (1). Your point (2) related not to cost, but to "...without having to depend on a cable down through some one else's property..." - which , as I said, does not seem to be a significant issue as regards the TN-S/TN-C-S comparison.

Kind Regards, John