Sizing of flylead to SWA

Sorry about idiot-level question. 

3 no Distribution circuits, 3 phase, one each in 4 x 16, 4 x 25 and 4 x 35mm XLPE/SWA and each with separate 16mm cpc alongside.  The armour will of course be in parallel with the external cpc but ignored for sake of R1+R2 etc. however doing belt-and-braces by using Earthing Nuts at both ends.  Other than treating the armour as extraneous part to be supplementary bonded, in which case presumably the 4mm (544.2.1) rule applies, is there any proper guidance on the size of the fly lead to these Earthing Nuts?  Cable sizes quoted above are required primarily from voltage drop considerations.

These circuits are taken from a larger distribution circuit, overall on TN-C-S with additional earth rod at the end of the primary Distrib cct, which is in the middle of a field.  There are no other parts or services that require Supplementary Bonding.

Thanks for your advice.