Adam, I have been following this thread with interest as I have a similar situation as the OP.

I have a house consumer unit in the hallway. A cable runs in a straight line from the middle of the back of the consumer unit straight through a double skin wall, then a ceiling - floor void and then a second double skin wall and then it is surface clipped around the garage wall to a secondary consumer unit which will be filled with RCBOs. So from my understanding of this thread I can fit a 45a MCB in the main consumer unit and connect the secondary consumer unit to that?

The cable is currently 6mm2 T&E and is more than 50mm from the floorboards and plasterboard......