Hi Brian,It is preferable that the supply to the garage isn't RCD protected, but that depends on whether the cable between the two is required by its type or installation method to be so protected. SWA doesn't need RCD protection, as long as the supply earthing isn't TT.
The cable is 6mm 2core and earth armoured swa. It will be earthed back to the main CU which is earthed to the incoming main as in the photo.
I have to mention though the I have installed a separate earth rod purely for off grid DC, solar panels, mttp charge controller and Battery which is in/on the garage. This rod is 20m away. Could that potentialy cause an issue?