sounds like you have it all under control - especially the DNO visit part - a good chance to pin them down on what they want to see.
Mike

the DNO may not like the two installations bonding together

Agreed, but keep in mind that if the two supplies feed the same building (or a pair of closely associated buildings) - the existing supplies are likely to be already solidly bonded together via bonding to common metallic pipes etc. Provided there's no evidence of overheating, that might give you an arguing point that things would be no worse than they are already, regardless of the DNO's latest rules. Of course the DNO might want to take the opportunity to improve things - but if that's their choice, there's an argument that it should be at their cost.

   - Andy.

AJJewsbury said:

Agreed, but keep in mind that if the two supplies feed the same building (or a pair of closely associated buildings) - the existing supplies are likely to be already solidly bonded together via bonding to common metallic pipes etc.

I don't think that connecting the two installation by metal pipes or steel-framed building is directly analogous, because unlike insulated wire, current may also flow through the ground from them. It all depends on local soil resistivity and how damp the conditions are around the pipe etc, as to how alike the two situations are.

So, yes if there are shared extraneous-conductive-parts, this might help the argument.

BUT - there's something else for Sparky Dan  to consider

In a lot of instances if the DNO actually know (or get to know) that there are shared extraneous-conductive-parts between two supplies in the same building, especially if there are large contact areas (e.g. steel-framed buildings) one of the two installations will not be offered a DNO earthing terminal. It may be against their conditions, so I'm not 100% clear what the situation would be here?

I agree Mike.  Arguably reg 12 of the Electricity at Work Regulations requires a single point of isolation/disconnection in this sort of installation given the likley knowledge and experience of those needing to cut the supply in event of an incident. 

Historically a number of DNOs refused to provide multiple supplies because of this and preceeding regulations but I am not sure what the policy(ies) is/(are) now. 

I have always been surprised that the need for considering a single point of isolation doesnt feature more in the regulations.

one of the two installations will not be offered a DNO earthing terminal.

Alternatively the DNO may make the N/PE split at a common point outside the building and run the two supplies with separate N and PE conductors from there to the two cut-outs. Certainly that seems to be the standard approach for some DNOs for new supplies into multi occupied steel frame buildings these days.

   - Andy.

AJJewsbury said:

Certainly that seems to be the standard approach for some DNOs for new supplies into multi occupied steel frame buildings these days.

Agreed ... some DNOs, but not all, and may depend on the configuration of the local network.