Therefore to avoid this risk the TT earth comes nowhere near the incomer until after its been though the RCD.

The reason is the TT earth will be too high impedance to clear such a fault by blowing the company fuse (probably too high to blow ANY fuse anywhere in the house come to that), and the upshot would be a live earth rod, live equipment chassis in the house, and a larger electricity bill than expected for all the ground current, but no automatic disconnection of supply would take place. The basic principle is that we don't want to be able to get a fault between the rod earth, and the supply live, that will not be isolated by the firing of the RCD. Perhaps its unclear what I was describing. These cables tend to be used for cross bonding of pipework and other extraneous metalwork in the installation that could concievably become live under fault conditions. The only time that smaller bonding cables are used is for supplementary bonding, and these can be 4mm if mechanically protected ie: installed in conduit or trunking, or 6mm if they rely on their sheath for protection. If there are any other Main equipotential bonds originating from this point, they too must be 10mm. These main bonds should be a minimum of 10mm and that means from the Stake to the Main Earth point should be 10mm, Bond to Gas, water and the Isolator should all be 10mm. If this main termination point happens to be in the Consumer Unit, then the Bond for the Isolator should come from the earth bar in the Consumer Unit. There should be a point in the Installation where the main equipotential bond from the Earth stake is terminated, it is from this point that a bond should be attached to the Isolator Earth Terminal.

Earthing isolator switch