How do I fix my seemingly improperly ground outdoor lighting circuit that trips my RCD when it rains?
After a bit more research, I understand that since the supply wire into the junction box is SWA steel armoured cable, I can use the armour as ground for the whole circuit.  Just need to check it is properly terminated in the breaker box so its earthed on that side , and also properly terminated in the junction box with an earthing plate I need to buy. I can then connect all the ground wires to the earthing plate and we should be good, no extra cable. 

I'm buying a bigger junction box that comes with earthing plates to terminate the SWA cable. I'll come back to this thread when all the components arrive and it's installed with some nice pictures.