vantech said:

Would I be better off TT’ing the potential socket

Perhaps not. Hot tub aside, the thing you have to ensure is that you can't touch exposed-conductive-parts of equipment connected to TT at the same time as exposed-conductive-parts (and really other metalwork connected to MET) of the PME.

If you can, then the installation is not compliant with BS 7671 (Regulation 411.3.1.1, second para).

With respect to the hot-tub, it may well be that the heater at least has exposed-conductive-parts in contact with the water, so there is possibly a PME risk (in an open-PEN event) too. Would have to be investigated.

I know some manufacturers of inflatable pools with heaters and pumps recommend they are unplugged when the pool is being used by people. How that works with 'bubbles" in an inflatable hot tub, I don't know without looking at one.

Thanks. 

If this socket is the last point on the circuit (or indeed, any outlet/electrical accessory) within reach of the tub, was placed in a TT island, with the accessories having built in RCD protection, would this suffice? 

The only potential issue I would then see is selectivity between the supply RCBO and the RCDs built into the sockets and FCUs that are on the TT. 

The TT Island would be at the end of the garden, probably 20 metres from the houses supplement earth stake. This supplementary stake is connect to the MET with the PME supply. 

If you only TT the last socket, then there's a risk that you'll have your TT island in touching distance of all the other lighting and any outdoor appliances that are still running on TN-C-S.

So the only real issue with this setup, is the fact a socket may be used for a hot tub. The other description using a 4mm radial is fine. 

These portable devices are generally installed by consumers, so any outdoor socket (or god forbid, extension lead) could be used.

Is there a specific regulation that prohibits using PME for a hot tub installation?

I would imagine the garden lights and electrical heaters will also be class 1 devices, so how far does this topic go? 

vantech said:

Is there a specific regulation that prohibits using PME for a hot tub installation?

Not specifically. In fact, PME is not prohibited for swimming pools either, although there is a NOTE in BS 7671 regarding earthing arrangements where PME is used for swimming pools (Regulation 702.410.3.4).

This article from July 2021 Wiring Matters discusses hot tub installations: electrical.theiet.org/.../

The considerations are unlikely to be too different for the inflatable or pop-up type hot tubs and heated swimming pools. However, accepted, anyone and everyone could just purchase and use one of those.

Yes, we've had inflatable pool (with filter, no heater) in our garden at home (PME) ... but I always made sure to unplug when the pool was in use, even with an SELV transformer for the filter pump ... the instructions said to do that without discussion of earthing systems anyway.