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. 

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.