also the supply can be repaired and it seems to say they can turn a TN-S into a TN-C-S without informing the user
Interestingly, when an isolator was being fitted to my supply a while back, I got the chance to see inside my cutout, and despite it looking a lot like TN-C-S from the outside, it is in fact TN-S as the feed is split con with no link between N&E. Interestingly I appear to be daisy chained to next door (as the supply cable goes off under my floor towards the neighbours, and through the party wall into their property), so who knows what their cutout looks like in terms of earthing arrangement.
When I asked the guy fitting the isolator, he said that although it technically is TN-S at my premises, they're told to treat everything as PME anyway, as it either has, or will be converted to PME at some point anyway. Not sure how much truth there is to this, and I don't have the kit to measure Ze, although I do have some paperwork (MWC) which came with the house, stating the Zs on the downstairs ring final was measured at 0.8.