See attached schematic 

Pedantically, the rod will effectively be an extraneous-conductive-part as far as the grid supply is concerned - to which it's almost certain PME conditions will apply - thus 6mm² will be too small - the minimum will be 10mm² (possibly more under some conditions). Physically the soil resistance around the rod will limit the current the bonding conductor can carry, so the 6mm²  will be perfectly adequate in practice, but BS 7671 (and associated PME requirement) don't recognise such subtlety, so if you want someone to certify it as complying with BS 7671 and/or building regs, that might have to increase a bit.

final approval / sign off 

Note that BS 7671 certification isn't a final exam kind of thing - more like continuous assessment - as the designer and installer have to sign as well as the inspector/tester. Some may be happy to 'oversee' you doing some of the work, but typically will need to be involved at each stage to do that.

   - Andy.

See attached schematic 

Pedantically, the rod will effectively be an extraneous-conductive-part as far as the grid supply is concerned - to which it's almost certain PME conditions will apply - thus 6mm² will be too small - the minimum will be 10mm² (possibly more under some conditions). Physically the soil resistance around the rod will limit the current the bonding conductor can carry, so the 6mm²  will be perfectly adequate in practice, but BS 7671 (and associated PME requirement) don't recognise such subtlety, so if you want someone to certify it as complying with BS 7671 and/or building regs, that might have to increase a bit.

final approval / sign off 

Note that BS 7671 certification isn't a final exam kind of thing - more like continuous assessment - as the designer and installer have to sign as well as the inspector/tester. Some may be happy to 'oversee' you doing some of the work, but typically will need to be involved at each stage to do that.

   - Andy.

AJJewsbury said:

Pedantically, the rod will effectively be an extraneous-conductive-part as far as the grid supply is concerned

I disagree. Perhaps, even more pedantically, it can't be an extraneous-conductive-part by definition, as it's part of the electrical installation. From Part 2:

Extraneous-conductive-part. A conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation.

It's relationship to the TN grid supply is effectively a supplementary earth electrode as discussed in Regulation 411.4.2.

AJJewsbury said:

PME conditions will apply - thus 6mm² will be too small - the minimum will be 10mm² (possibly more under some conditions).

I do, however, fully agree with your view on sizing the conductor connecting it to the installation ... it will see the same currents as an extraneous-conductive-part even though it's not one, and therefore where PME conditions apply, the minimum cross-sectional area ought to be according to Table 54.8 of BS 7671 in relation to the grid supply