It will tend to err even more on the side of caution than it already does with a PME system. (usually a calculation shows you do not need as much metal as table 54.7 suggests, but it is safe to assume you do.)

By how much the main earth  may be reduced rather depends on the earthing system you do have, as this determines the potential fault current. which is the figure you insert into 543,1,3 to calculate a more accurate value of minimum CSA.

The parallel armoured cables thing is interesting as you should still earth both armours, but maybe only one of them at both ends, depending how this is laid out. In really large sizes it is often more effective to use singe armoured cables trefoiled or quadrifoiled, but then one has to think hard about the currents that circulate in the armour.  And a 185mm SWA will be a right so-and so to thread into anywhere.

Mike,

We are not bringing in the incoming supply, but we need to allow for an indicative design that will be designed and installed by others. Even although the SWA will be earthed we are allowing a direct connection from the Main Earth Bar to the DNO Earthing Point. I know adiabatic calculation will be the way forward at the design stage, but for the indicative design I was hoping to use tabulated figures.

We are not bringing in the incoming supply, but we need to allow for an indicative design that will be designed and installed by others. Even although the SWA will be earthed we are allowing a direct connection from the Main Earth Bar to the DNO Earthing Point. I know adiabatic calculation will be the way forward at the design stage, but for the indicative design I was hoping to use tabulated figures.