Run a 6mm copper bar via a terminal block from the inner wall to the outer. Have yet to connect the rod to the external earth.
July 4th Have used old stiff 7 strand 3mm earth wire and connected MPPT, Battery and Inverter to earth block.
July 11th I have finally soldered the internal earth rod to the external grounding strip, it is not pretty and I could have a) made a better job of it by protecting the wood and mains cable or b) made a nice copper clamp to join them ~ a future mini project.
To see a better extent of the scorch marks view ~ Scorch Extent