Tripping RCD on Night Storage

I've just been called to a customer who can't reset his RCD on the night storage CU. When I had a look, it would only reset if the main switch in the main board was closed. As soon as I opened the main switch on the main board then the RCD on the Econ 7 CU tripped.

This is a fault that has just appeared. Investigating further - it is the neutral that trips it (but only when the main switch on the main board is closed). Any ideas?

What testing have you done??

+1 to the above. You won't find the fault without some testing.

Because circuits on one CU are affecting what happens on another, It must be the circuits linked in some unintentional way so shouls be easy to find.

Anyone borrowed  a  neutral ? 

Has he or anyone else done any work in the property in the last few days?