Hi Chris, & welcome!
As Texas has said, the neutral on the load side of the RCD could be the problem. Is there any way that it could have been cross-connected with another neutral somewhere?
The other possibility that springs to mind is simply a faulty mechanism in the RCD. You said it works O.K. on the bench, but I'm assuming that on the bench test the device is laying on its back rather than being mounted vertically. I've found faulty units that will only latch when held a certain way.
