Quick update to this thread, I've had another look at the way everything is wired and it turns out both this PSU and a servo controller is on the same AC supply, fired off the same main switch. There's an AC filter on the AC supply to stop anything flowing back into the AC line, but there isn't really anything protecting the PSU on the AC side. No idea whether any damage can happen this way when the servo controller is switched on / off, but I thought I'd check.

Also, just noticed that if I leave the entire rig powered for a long time (ie. over 10minutes), the PSU eventually produces around 40VDC which is fairly steady. Could this be related to a dead capacitor perhaps which is simply taking a very long time to charge? again, no idea if this even happens in a PSU, so just throwing an idea out there.

T.