Is the consumer unit in the garage the only one in the property ?
If there are two feeds into the junction box ,are the lights on their own circuit / MCB ,and the sockets on a different circuit /MCB ?
I suspect your drawings are not a true representation of the actual wiring. What electrical testing equipment do you have ?
You state that the socket circuit and lighting circuits are in some way interconnected. If you turn off the sockets MCB ,does the lighting circuit still work ? Do you consider the cross connection to be on the line ,or neutral , or both ?
It's very unlikely as the two circuits are protected by different RCD's.
In fact nothing would work !
Are you sure the garage sockets are on the kitchen socket circuit ( MCB marked kitchen sockets )?
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