I get what you and Lurch are driving at; the "contradiction in terms" was inaccurately forceful. But the OP did seem confused about whether their installation was TT or TN-C-S, or some deliberate or accidental hybrid inbetween.
I can quite see that PME in an urban setting, with genuinely Multiple places where the N and REC-supplied-E are bonded, and relatively short distances between local substation and consumer with several points along the way where the bond to physical earth is made, is a Good And Safe arrangement which provides a low-impedance fault path in all but the most bizarre multiple-fault circumstances. And by far the greater contribution to a low phase-to-installation-E fault impedance is the (reliable) bonding of the installation-E to the supplier's N. (OK so far? Pull me up if I'm all washed up here!) And the significance of the bonding to "local" E is (it further seems to me) to reduce any dangerous disparity between the potential attained by the installation-E-bonded metalwork (both Extraneous and Exposed ;-) and locally-referenced E, be that a damp cellar floor, water/gas pipes, etc, right? (Hence the more rigid main equipotential bonding requirments in a PME install.)
Where that reasoning seems to me to be shakier, though, is a supplier making PME available on an overhead supply. There, I'm imagining more risk of (selective) conductor breakage; greater distances to substations; higher impedance paths back to said substation/nearly-local-pole-mounted transformer; all conspiring to make the likelihood of the supplier's N, and hence their provided installation-E in a PME system, floating some significant distance away from "local" E. For such an installation, the relative simplicity of TT as a concept seems to win in my under-informed brain over the supplier's provision of a TN-C-S earth.
I really would like to understand this, not just theoretically but because it's just the situation in a scout house I feel partly morally (luckily not legally) responsible for. There we have an overhead supply, treated at the time of a thorough and competent upgrade about 20? years ago as TT (oh, it was a joy last summer when the rain and rodents got into a couple of the circuits and the 100mA whole-installation breaker kept tripping, expecially in the small hours so setting off the fire-alarm's mains-loss warning... but I digress). The transformer up the pole, which supplies that house's LV, was replaced by the REC more recently, and the pole now sports a splendid PME label. So it's possible that at the next overhaul the idea of taking the supplier's E might be seriously considered, and I'd like to know what that would mean in practice in this intermittently-occupied, hired-out-to-various-groups, occupied by groups of kids as young as 7, with dippy leaders who don't know one end of a CU from the other (fantastic as they are with the kids, mind), electric-showers-in-outbuilding, etc. etc. house!
Stefek