Thanks everyone for the responses, this has led to quite a good discussion and lots of poitns have been raised,

It sounds like there is no way to comply with both BS7671 (to the letter) and the ESQCR without introducing a galvanic isolation transformer either upstream/immediately downstream of the UPS depending on the UPS configuration. 

Paralleling both NE earth links makes the installation TN-C-S, albeit very briefly, and this goes against the ESQCR as there is a PNB within the consumers installation.

The ‘best’ option would be to use a fast switching 4P static switch upstream of the UPS to handle the rapid changeover of supplies. Although this is not compliant with BS7671 to the letter and would depend on whether the downstream supplies could accept the momentary disconnection of the neutral.

Are you taking the main supply from the DNO at LV or HV?

If you are taking the supply at HV (i.e. you own the transformers and generators), then see Fig 9A of BS 7671.

If you are taking the supply at LV, then you need a different design solution … but you do anyway, as you can't rely on the distributor's means of earthing for the UPS in any case, see Regulation 551.4.3.2.1

Thanks Graham,

The supply will be taken at HV from the DNO and then fed through a private network. The LV mains is then derived from a local distribution transformer and tied to earth via the NE link in the main LV switchboard.

“If you are taking the supply at HV (i.e. you own the transformers and generators), then see Fig 9A of BS 7671.”

I am struggling to see how figure 9A can be applied when there are switching devices between the two sources. 

Where would the single NE link shown in figure 9A go? If it is within the main (non-essential) portion of the switchboard, then on changeover to generator (by 4 pole as required by BS7671) the neutral earth link would be lost. 

During the changeover the UPS would be allowed to ‘float’ as there would be no neutral-earth link upstream of it. Although this may only be momentary, does this not contravene BS7671?

Because it's a not public LV supply (you own and operate the transformer), there's no need to “transfer” the Neutral. This is discussed in Regulation 551.6.2.

If you were taking the supply from the DNO at LV, it would be recommended to switch the neutral, and you would also need an independent means of earthing. In this case, transfer to generator with its own earth …  but then there is a period when the UPS is floating and perhaps not safe. In that case, a transformer would be required immediately upstream of the UPS to establish a new neutral, or use a UPS with integral transformer.

The transfer switch does need all of the line conductors to be switched, however, to prevent the generator operating in parallel with the public supply through the transformer.

If you look at Page 195 of the Commentary on the IET Wiring Regulations (2015 Edition) you will see in Figure 7.11 what the objective of the Regulation is. It's to prevent circulating neutral current.

If you have an arrangement in which Neutral currents cannot circulate - for example you only have one feed downstream, and the generator comes into the main switchboard before supply to the essential board, that can be avoided.

If your arrangement does not permit this, then you will need a transformer for the UPS as per a public supply.