300 a breaker i think, 3 phase supply and 400a across all 3 phases
I would be inclined to confirm this, its not really the way 3ph current is usually expressed, it also seems overkill fitting a 300A front end because of the implemented costings.
3ph 400amp is usually 400amp per phase
If what you say is correct then you are telling us 133amp per phase but with a 300amp front end protection this would all be ringing alarm bells as to what you exactly is required as i would suspect 3ph 160amp/200amp system would vastly reduce costs of both materials and installation while still giving a comfort zone.
So il re-iterate we have a serious lack of info especially regarding nature of loading, also as already mentioned your opening post was requestion info completely different to that of which you described later in the thread.
Im can only assume this is well out of your comfort zone and you may be panic questioning as some questions you have asked are clearly explained within the regs..... we cant know the additional earthing needed until we know:-
(A) The nature of the load (dedicated circuit, sub-mains(multiple ways)).
(B) The correct specs of what your asking which will give us a cable size, then we can attain if the armour is acceptable as the earthing alone.
(c) Does anthing need Bonding (Gas/Water steel etc) as this will need to be included when working out earthing.
I would personally in your case get someone of experience on board as a little mistake in design could be costly and IMHO we can only head you in the correct direction but a to give you a detailed answer would require a detailed spec and/or onsite visit.