I'm sure the more prolific forum members will be on, shortly, to elaborate further - obviously if my understanding is incorrect, I'm sure to be corrected soon:

1) Does it have any metal bits going into the ground that require bonding? Gas, water, metal framework etc

2) If so, one option - is to run a cable that is capable of (Sized large enough) being both main earth and bonding conductor - you don't need to add these together or anything, just pick the greater of the two, and the sub distribution earth size needs to be at least that big. Export the TNCS without worry. 

3) If there's no metal bits going into the ground then there's no worries about bonding sizes, just main earth sizes followed by CPCs in the out building as normal final circuitry and RCD protection as normal. Export the TNCS away. No Problems.

4) You say, but what about EVs? Why are they so different? - My understanding is - they're a nice conductive metal framework that hovers above ground - insulated by rubber wheels- and under the wrong circumstances with a lost N on the supply side, can send current via the earth, and the voltage - instead of being at or near zero becomes a floating voltage at some level (I cant explain this but something along those lines)  (Including that earth out to the car) and it can sit there waiting for the user to come and touch the car body..... and Zap..... oh no........The Matt-e device then monitors the N-E and if it rises, cuts the power. Something along those lines anyway. I feel a clarification coming my way.....

3) You certainly can TT it too. 

goldenboy1818 said:

couldn't remember all details on exporting pme outside

I am not sure that the function of the building is relevant - it could be a detached garage, for example. No problem with "exporting" TN-C-S as such, but if a hot tub needs a TT earth, it may be easier to TT the whole building.

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. 

I get the impression that on the mainland you cant do anything outside without worrying about the DNO neutral becoming open circuit. No doubt all the fuss over EV installations has percolated through to the wholesalers counter.

It seems that despite the statement in Regulation 114.1 (second paragraph, see extract below) and despite the use of the word "shall", some are determined to tie themselves in knots every time a cable breaks out of a main building. 

On a PME supply, if the neutral is chopped through in the street, there should always be a connection to earth somewhere.  But with all the neutral current going through it, that doesn't mean that it's anywhere near 0V compared with a screwdriver in the flower bed.

It seems that despite the statement in Regulation 114.1 (second paragraph, see extract below) and despite the use of the word "shall", some are determined to tie themselves in knots every time a cable breaks out of a main building. 

Ah, but be aware of the dictionary meaning of the word "permanent" - unlike common usage it doesn't actually mean "forever" or even for a very long time, and certainly not reliable. It simply means for an undefined length of time (compare with temporary which means for a defined length of time). Something temporary could easily outlast something permanent - just as long as you didn't know initially how long the permanent thing would last.

   - Andy.

You must be using a different dictionary on the mainland!

AJJewsbury said:

Ah, but be aware of the dictionary meaning of the word "permanent" - unlike common usage it doesn't actually mean "forever" or even for a very long time, and certainly not reliable

Not sure about that, Andy. OED says, "Continuing or designed to continue or last indefinitely without change; abiding, enduring, lasting; persistent. Opposed to temporary."

However, caselaw may have different views depending upon the context.