I expect you're right on your advice re using a qualified electrician but perhaps you're not familiar with modern E7 meters. It has six terminals: L&N in , L&N out and two much smaller connectors (red wires in my pic). These two terminals go closed-circuit during E7 time. They do not go live during E7 time. Hence I am unsure how they would be connected through my RCD or even through an additional RCD.