If the hot tub was located on bare ground or grass, i think i'd be also installing a rod, even if it was a TN-S supplied installation. Your still going to get a potential difference between true earth and the DNO TN-S earth.....
As for the Regs, i think they state somewhere, that a TNC-S supply can be used, for supplying hot-tubs and the like provided an earth electrode of a certain value is also provided.... I think that value was around 20 ohms, but i wouldn't swear to it...
Nothing like PME E54, i very much doubt you would feel any sensation on a TN-S.
As for an Electrode on a PME, for it to be any use you would be looking at very low single figures.
A lot of these Hot Tubs are class II as well.