Volex Consumer Unit

Hi,

I've just installed a new consumer unit in my garage, as before all the electrics have been spurred from a socket brought through from the house. I've used the volex garage consumer unit from screwfix.com

. I've spent the past 2 days trying to figure out why the damn thing keeps tripping out when I put a load on it. I've connected up a light, with 2 switches, one either end of the garage, when you turn flip the RCD, and the MCB for the lighting circuit, the RCD trips out. I've tried a simple circuit, with just twin and earth, and a light fitting, no switches, and it still trips out. I have then connected the incoming power to one of the other MCBs, by passing the RCD and it works fine. I've checked and double checked all the wiring, and the incoming power, everything is earthed. Could it be a faulty RCD? I read a thread posted to this group back in March and someone mentioned something about making sure the Neutral from the mains was going through the RCD as well as the Live, and not straight to the bus bar. However I can not see where this would go in this consumer unit, there is only one N bus bar and it is not connected to the RCD in anyway, so I can't see how it would run through it.

Any help greatly appreciated.

Cheers

Ben