RCD tripping when socket circuit powered up?

I have a dual RCD consumer unit which I have a fault with an RCD tripping when any of the two socket circuits are powered up.
I did find a neutral to earth fault at one of the sockets on the downstairs ring main which has been rectified and IR test proves this ok.
What I found was that when I powered everything back on for that side of the board the RCD protecting the circuits at this side of the board was ok , did not trip.
But when I closed the RCD that protects the other circuits on the other side of the board it tripped.
I opened the MCB for the socket circuit that had initially been faulty reset the RCD that had tripped then closed the MCB for the sockets and again this tripped.
I reset the RCD again and then one by one closed each MCB for the circuits protected by this RCD and the only circuit that tripped this on this side was the upstair socket circuit.
An IR test on this circuit was found to be ok.
Would the problem be neutrals that are possibly tied together somewhere , didn't have time to continue looking in to this at the time.