Specifying a new LV main board

well the '1 ' got added to the area codes in 1992-1993 ish. So its done at least the first  30 years of its  service ;-)

The choice of form thing is all about how easily sections can be worked on, while the rest of it is kept energised - the higher numbers having more isolatable compartments, taking up more space and generally being more bosky..

https://www.sandfordelectrical.co.uk/guide-to-form-type

clarifies this quite well.
It  is not the only consideration though - to work on something also needs enough room to get tools and personnel into position safely.
Sometimes a small number of fused submains each to another panel elsewhere is easier.
How much of the circuitry beyond the main switchgear is being kept and is there a requirement for earth fault detection ? - As you mention clean earths it may be a funny site with lots of filters, and therefore current wiring in the earth even when operating normally. This will determine the no. and complexity of the outgoing ways.

Dual supplies is not that uncommon, but needs more thought, and again earthing, or at least the path for fault currents to flow, can get messy.
Beyond all but assuming the simplest like  for like swap, you will need to involve someone who can answer these questions. it is beyond a simple response.

Mike.

We have a 3rd party consultant to act as our AP but when I asked him a few questions his first response was sounds like you need a performance spec..... I'd rather be more informed about what I'm asking for hence the questions. Its primarily a board swap with all circuits retained. I'm looking to use MCCB's rather than fuses.

Hmm. Perhaps you have the wrong sort of consultant, or the wrong contract, if they are  not involved in requirement capture and you are not that happy doing it.

Fuses. You may not be aware, but in high PSSC situations, there  can be a significant advantage to have a "death or glory" fuse, or 3, near the origin of any installation of the sort of current you imply, as fuses operate faster at higher fault currents in a way that beyond certain point, breakers with moving parts don't. Then the breakers do not discriminate well with other breakers downstream as it were. In effect the fuse (that never blows really ) will act as a let-through energy limiter, perhaps allowing smaller, lighter and cheaper switchgear down stream of it.
(similar arguments allow domestic 6kA breakers behind a 100A intake even if the PSSC before the intake is more than 6kA )
Moving down the branches of the distribution tree towards the loads, MCCBs and then MCBs are entirely sensible.
Mike

Since yesterday the electrical contractor we regularly use has sent me through some panel drawings on other buildings. A Form 4 type 5 has been used on a manufacturing building and a 4/2 on another office type building probably similar to the one being discussed here. I note they are using fuses on both. I imagine the rationale being the effect you describe.

Its a new role I'm in where traditionally my employer has relied heavily on contractors and consultants for advice, something I'm in post to affect positively.  Its predominantly mechanically heavy hence the bias but none the less positively influencing and challenging electrical design also sits in my remit, the consultants are perfectly able to write performance specs but I want to be able to understand what I'm getting and render an opinion.

Whilst I have much learning to do on MV/LV the age of these old parts mean we are on borrowed time really and if they fail we are possibly in deep trouble. So I appreciate the advice you are giving.

I did read the categorisation for the forms before positing here and read that very page and whilst I superficially understood the text I was less able to translate that into a practical decision on which type, type 5 seemed appropriate for the application I have in mind.

Please don't think I'm trying to tell you your job ! This size of stuff is  not the sort of thing I get much involved with either, except when things I make need a connection to it, and your earlier post did read like you had been dropped in it rather.
Anyway, happy to explain stuff where I can - though there are others who frequent here with bigger tools who will have more useful practical advice about everything down to lug sizes and bend radii...

Things to think about  in addition to space and cost, is how much can be left on while changes to the supply to another area are being made - as it sounds like the sort of thing that may be needed.

Access and labelling are then even more, important especially with dual supplies. Well annotated diagrams are also something I wish other folk did more often.

The rest is then more or less standard electrical, just with bigger numbers. By which I mean  the hierarchy of breaker values and how they discriminate with faults both near and far down the tree, and perhaps, a similar tapered approach to the handling of earth faults in a time delayed and graceful way nearer the origin than the 30mA RCD protecting the sockets.
Mike

Please don't think I'm trying to tell you your job ! This size of stuff is  not the sort of thing I get much involved with either, except when things I make need a connection to it, and your earlier post did read like you had been dropped in it rather.
Anyway, happy to explain stuff where I can - though there are others who frequent here with bigger tools who will have more useful practical advice about everything down to lug sizes and bend radii...

Things to think about  in addition to space and cost, is how much can be left on while changes to the supply to another area are being made - as it sounds like the sort of thing that may be needed.

Access and labelling are then even more, important especially with dual supplies. Well annotated diagrams are also something I wish other folk did more often.

The rest is then more or less standard electrical, just with bigger numbers. By which I mean  the hierarchy of breaker values and how they discriminate with faults both near and far down the tree, and perhaps, a similar tapered approach to the handling of earth faults in a time delayed and graceful way nearer the origin than the 30mA RCD protecting the sockets.
Mike