EICR with a house connected to a detached garage via a submain, part of which is PVC T+E and due to the route it takes through the house requires RCD protection.

I need to change the RCBO at the house end to a bidirectional one, luckily MK have just started shipping a suitable device. This gives me protection when power is flowing from house to garage.

But now looking at when solar is generating power, the supply from the solar is just protected by an MCB. Therefore when surplus solar is feeding the garage and submain to the house, from what I can see the garage circuits and submain do not benefit from RCD protection.

Section 826 covers overload protection well but doesn't cover RCD protection. I know that good practice is to install so that RCD protection is not required, but I am dealing with a legacy system.

My initial thought is to add a bi directional RCBO where the solar feed is connected to the garage board.

Will this then provide suitable RCD protection?
Any risk of interactions / imbalances causing nuisance tripping

I  believe that once the house side RCBO trips the inverter would stop seeing a supply and shut down, I assume this won't be quick enough to provide shock protection and is unacceptable for other reasons.

I believe the ideal solution is to have solar connected to the house DB direct by a means that does not need RCD protection, this isn't financially viable, would make more sense to decommission the solar system.

There is a possibility that the sub main from house to garage is converted to a buried SWA cable and the problem goes away, but this may not happen and looking for cost effective acceptable solution with existing cable run.

As some background the solar system is 10 to 15 years old, actually mounted on some stables which the house owner doesn't have have easy access to.