Above all sounds good to me.

It's not just EVs - the original prohibition was on caravans (and boats). Same safety concerns - earthed metallic object outside, often on (or next to) bare wet ground, so if the earthing system happened to be at a potential significantly different to that of true earth, fatal shocks could easily result if someone touched the caravan (or more likely positively grabbed hold of some part such as a door handle or hitch) while standing on the ground (especially if not wearing insulating shoes). And PME (indeed any TN-C-S system) has one particular single fault failure mode (broken PEN) that can leave the earthing system at anything up to full line voltage indefinitely.

Indoors in a building of mostly electrically insulating construction (especially the floor), it's easy to construct an equipotential zone, so there's not much sock risk even if the earthing system is at 230V - as there will be no potential differences between accessible parts.

It's not as simple as always avoiding PME'd metalwork outdoors though - there are risks both ways (if you TT for instance, you have to rely on RCDs and local earthing electrodes, both of which have a comparatively high failure rate) so often it's a cause of weighing one risk against another. Things that are not routinely touched - e.g. steel lampposts or outdoor heat pump units may well be safest (as in least unsafe) kept on PME.

Additional electrodes on PME systems can help - especially where the load is relatively small. A 20Ω electrode for instance can easily keep a 1A lamppost below 20V in a broken PEN situation. But things gets harder as the load increases - for a max 100A single phase load for example you'd need something like a 0.5Ω electrode to keep things below 50V - and 0.5Ω is pretty much impossible to achieve reliably in practice (at least on a domestic scale). Still if each property had an electrode of some sort, and there were several properties downstream of the broken PEN, the cumulative effect can still be useful, especially on 3-phase distribution systems where loads on different phases tend to cancel out to some extent, reducing the N current and resulting voltage differences, while the electrode continue to add up (well, in the way that resistances in parallel do).

The DNOs have certain length limits (often 40m) between their closest electrodes and the consumer, but that's more to do with the voltage differences along the PEN conductor - BS 7671 has no equivalent restrictions inside the installation. Note that once the N and PE conductors are separated out (as they will be inside the consumer's installation) N current don't then affect the voltages differences along PE conductors.

   - Andy.

Above all sounds good to me.

It's not just EVs - the original prohibition was on caravans (and boats). Same safety concerns - earthed metallic object outside, often on (or next to) bare wet ground, so if the earthing system happened to be at a potential significantly different to that of true earth, fatal shocks could easily result if someone touched the caravan (or more likely positively grabbed hold of some part such as a door handle or hitch) while standing on the ground (especially if not wearing insulating shoes). And PME (indeed any TN-C-S system) has one particular single fault failure mode (broken PEN) that can leave the earthing system at anything up to full line voltage indefinitely.

Indoors in a building of mostly electrically insulating construction (especially the floor), it's easy to construct an equipotential zone, so there's not much sock risk even if the earthing system is at 230V - as there will be no potential differences between accessible parts.

It's not as simple as always avoiding PME'd metalwork outdoors though - there are risks both ways (if you TT for instance, you have to rely on RCDs and local earthing electrodes, both of which have a comparatively high failure rate) so often it's a cause of weighing one risk against another. Things that are not routinely touched - e.g. steel lampposts or outdoor heat pump units may well be safest (as in least unsafe) kept on PME.

Additional electrodes on PME systems can help - especially where the load is relatively small. A 20Ω electrode for instance can easily keep a 1A lamppost below 20V in a broken PEN situation. But things gets harder as the load increases - for a max 100A single phase load for example you'd need something like a 0.5Ω electrode to keep things below 50V - and 0.5Ω is pretty much impossible to achieve reliably in practice (at least on a domestic scale). Still if each property had an electrode of some sort, and there were several properties downstream of the broken PEN, the cumulative effect can still be useful, especially on 3-phase distribution systems where loads on different phases tend to cancel out to some extent, reducing the N current and resulting voltage differences, while the electrode continue to add up (well, in the way that resistances in parallel do).

The DNOs have certain length limits (often 40m) between their closest electrodes and the consumer, but that's more to do with the voltage differences along the PEN conductor - BS 7671 has no equivalent restrictions inside the installation. Note that once the N and PE conductors are separated out (as they will be inside the consumer's installation) N current don't then affect the voltages differences along PE conductors.

   - Andy.

So with no metal parts in this wooden summerhouse I think the tncs would be fine exported

Would it be worth adding a earth rod in and connecting as a back up alongside the pme earth supplied from the 10mm swa ?

I don't think there's any harm in installing in an earth stake, but you run the risk of hitting underground services and pipes and drains etc. I wouldn't bother.