gkenyon: 
 

There is no problem having the DNO's earthing terminal connected to MET at the same time as your own earth electrode (just like extraneous-conductive-parts). In fact, a consumer's supplementary earth electrode is recognized by BS 7671. See Fig 3.9 and the descriptions below it.

What you MUST NOT do is connect N to PE within your installation (downstream of the DNO's service head), without disconnecting the distributor's neutral first - regardless of whether the supply is TN-C-S (PME), TN-S, or TT. This is not only an issue for protective devices, but also potentially contravenes ESQCR.

Ah I see, so you’re saying the order of change over needs to ensure line conductors are broken before the N-E link is made by the inverter, and then broken first before the DNO supply is reinstated?

I would hope (wrongly or rightly) the inverter has been designed to take care of this, where it’s “backup” output shuts down, breaks the N-E link and then reinstates what is DNO supplied/synchronised power onto its backup output. 

I can see some inverters on the market have separate automatic change over devices using sets of contactors, however they don’t seem to address the issue of the N-E bond. 

This is something from Solax - install guide at the bottom of the page.

https://www.wattuneed.com/en/mounting-and-accessories/21133-x1-eps-box-solax-0768563817090.html

My thinking here is that actually this device breaks the grid input before it allows connection of the inverters backup supply. I think a simple relay could be added here to make a E-N link on switch over. 

gkenyon: 
 

There is no problem having the DNO's earthing terminal connected to MET at the same time as your own earth electrode (just like extraneous-conductive-parts). In fact, a consumer's supplementary earth electrode is recognized by BS 7671. See Fig 3.9 and the descriptions below it.

What you MUST NOT do is connect N to PE within your installation (downstream of the DNO's service head), without disconnecting the distributor's neutral first - regardless of whether the supply is TN-C-S (PME), TN-S, or TT. This is not only an issue for protective devices, but also potentially contravenes ESQCR.

Ah I see, so you’re saying the order of change over needs to ensure line conductors are broken before the N-E link is made by the inverter, and then broken first before the DNO supply is reinstated?

I would hope (wrongly or rightly) the inverter has been designed to take care of this, where it’s “backup” output shuts down, breaks the N-E link and then reinstates what is DNO supplied/synchronised power onto its backup output. 

I can see some inverters on the market have separate automatic change over devices using sets of contactors, however they don’t seem to address the issue of the N-E bond. 

This is something from Solax - install guide at the bottom of the page.

https://www.wattuneed.com/en/mounting-and-accessories/21133-x1-eps-box-solax-0768563817090.html

My thinking here is that actually this device breaks the grid input before it allows connection of the inverters backup supply. I think a simple relay could be added here to make a E-N link on switch over.